What if Donald Trump becomes president?

Most likely, he'd be ineffective. Possibly impeached. But also possibly transformative.

A President Donald Trump still remains an unlikely scenario. But it's now December, and he's still atop the Republican polls. What if he somehow becomes president?

The simple answer is that most likely he would make an ineffective and terrible president, because he has no real understanding of what being president involves. And in all probability, the damage he can do is limited by the basic constitutional and administrative designs of the US government, which restrict the powers of the executive.

Sure, the president may serve as commander-in-chief and be able to command the bully pulpit. But Congress passes the laws and allocates funding for the executive branch. And a massive civil service bureaucracy has a will of its own — and the kind of job security that The Apprentice never had to deal with.

As President Truman said of Dwight Eisenhower in the 1952 presidential election, "He'll sit there all day saying do this, do that, and nothing will happen. Poor Ike, it won't be a bit like the military. He'll find it very frustrating." So, to rephrase Truman: Poor Donald; it won't be a bit like running his own company.

Trump's policy promises run the gamut from unrealistic distortion to infeasible bluster. He's managed to stay vague by relabeling vagueness as the core of his future negotiating strategy. So predicting a policy agenda is a pointless exercise.

Still, here are three potential scenarios for the general dimensions a Trump presidency could take, ranked from most probable to least probable (with my impressionistic probabilities attached, and leaving aside a 5 percent possibility of something else).

1. An ineffective president at war with Congress (probability: 65 percent), possibly impeached (probability: 10 percent)
2. A standard-issue Republican (probability: 25 percent)
3. A populist maverick realigner (probability: 5 percent)

Each of these scenarios involves some guess as to how Trump and other key players in Washington would respond to each other. As presidential scholar Richard Neustadt once noted, the president's power depends first and foremost on relationships. Contrary to the popular view (which political scientist Brendan Nyhan has dubbed the Green Lantern theory of the presidency), the president cannot just make things happen by sheer force of will and personality. A Trump presidency (or any presidency really) will depend both on what he does and on how others respond.

Scenario 1: An ineffective president at war with Congress, possibly impeached

The first and most significant decision point in a Trump presidency will involve the question of detente with Republican elites, especially those in Congress, who are increasingly panicked and paralyzed about a Trump presidency

Most likely, whatever detente occurs will be limited. Try to imagine what happens when Senate Majority Leader Mitch McConnell, House Speaker Paul Ryan, and Trump all sit down together. Presumably, Trump would come in with his usual bluster, demanding things be done his way or no way: McConnell, Ryan, and their cohorts can go along or get insulted publicly. McConnell and Ryan are not easily bullied. They are used to getting what they want, too.

Trump's chest-thumping, Washington-is-run-by-corrupt-idiots bravado may work well on the campaign trail, especially given the sour public mood. But if he arrives at 1600 Pennsylvania Avenue as an outsider without bridges even to his own party leaders, he'll be following the model of Jimmy Carter, who also came to Washington as an outsider who thought he knew better than the people who had been here for decades and left as a failure of a president.

In 1977, as Carter's presidency began, Democrats had solid majorities in both the House and the Senate. Yet Carter failed to achieve much of his agenda for the simple reason that senators and members of Congress had egos (surprise!) and didn't appreciate being talked down to and told what to do — even if they were fellow Democrats. In Neustadt's timeless words, the US is a "government of separated institutions sharing powers." The president is not a unitary actor. Without existing strong networks of allies in Washington, it's impossible to be effective as an executive.

Moreover, without 60 Republican votes in the Senate (and it's highly unlikely Republicans will win 60 Senate seats, even if Trump wins), there are limits to what can be accomplished. And Democrats will have every incentive to make it difficulty for a President Trump to pass his agenda.

Yes, the president does have the power of to command public attention. But considerable evidence, much of it marshaled by political scientist George Edwards, has demonstrated that the power to actually move both public opinion and legislation is very, very limited.

Additionally, Trump will have his own bureaucracy to deal with. In a government of career civil servants with very good job security, many of whom would presumably be at odds with a Trump administration, there are plenty of ways to stall and ignore orders from the top. There are also complicated logistics to getting anything done. Say Congress did appropriate funds for building a massive wall along the southern border, as per President Trump's campaign promise. How long would it take? Imagine the contracting and subcontracting difficulties, the permits, the full federal bureaucratic rigmarole. Poor Donald; it won't be a bit like running his own company.

Finally, it's important to remember that the US is a federal system, in which state governments have considerable authority. As Obama discovered in the implementation of the Affordable Care Act, it's not easy to get states to make major changes.

Impressionistic probability of this scenario (conditional on Trump winning): 65 percent

Within the ineffective president scenario, there's a small but reasonable possibility that a Trump presidency ends in impeachment and/or resignation.

In all likelihood, if he can't get anything done, a President Trump would become more and more frustrated with both Congress and his own bureaucracy. Annoyed by constitutional limits, he could start doing things his way, playing fast and loose with the laws.

Scandals dog all administrations. The potential for scandal in a Trump administration seems especially HUGE, given his desire to accomplish big goals and his impatience for people who stand in his way.

If presidential-congressional relations grow frosty under an increasingly unpopular Trump administration (as they likely would), even Republican leaders might see political gain in ousting him. Or a frustrated president Trump could just resign in a fit of outrage.

Impressionistic probability of Trump being impeached (conditional on Trump winning): 10 percent

Scenario 2: A standard-issue Republican

A second scenario is that as it becomes clear that Trump is going to win, Republican elites make common cause with him. They agree to get behind him. He agrees to tone down some of his rhetoric. And, most importantly, he agrees to surround himself with the standard fare of trusted Republican advisers, some of whom he appoints to key Cabinet positions.

The case for this scenario is as follows. Trump appears mostly just to crave attention and prefers showmanship to details. Once he turns his attention to the actual responsibilities of being president, he'll appreciate the value of delegation.  He'll just appoint the "smartest people" to run things, then thrive on the speechifying and other perks of the presidency. And since he cares more about the spotlight than he does about policy, he could be convinced that going along with the standard Republican agenda makes the most sense, since he'll have the most institutional support and therefore the most likelihood of a successful presidency. If this happens, he could be a lot like Reagan, using his own personality to make the case for policies developed by his advisers and allies.

If Trump is as good a dealmaker as he claims to be, he'll grasp that presidents who succeed are those who recognize how weak their own hand is, rather than overplaying it. To get what he wants (presumably continued public love), he'll have to let others feel like they are running the show.

The big difference between this scenario and the first scenario is that in this scenario, Trump acknowledges some of his own limitations and also gives up on some of his outsider, disruptor mojo. This scenario requires him submerging his ego somewhat and trusting others (which is why it seems less likely).

This scenario also involves Republican elites deciding before it's too late that it's better to join Trump than to beat him, and also having some ability to work with him and persuade him to fit the standard Republican mold. This requires Republican elites overcoming considerable antipathy towards Trump.

Impressionistic probability of this scenario (conditional on Trump winning): 25 percent

Scenario 3: A populist maverick realigner

A low probability, but still intriguing, possibility is that Trump plays it as a true populist maverick, finding common cause with outsiders in both parties against Washington insiders on poll-tested issues like trade, Wall Street regulation, Social Security, money in politics, and perhaps a few others. In short, he might see a political advantage in a set of issues where public opinion is opposed to the general consensus of moneyed elites.

If this happens, it could start a political realignment. Led by Trump and bolstered by Tea Party support, the Republican Party could become the party of populism, appealing to middle-American voters who have been marginalized in the current two-party alignment. Business elites would migrate to the Democratic Party, which would become more and more the party of coastal elites. My New America colleague Michael Lind has envisioned this kind of realignment.

This seems like the least likely president Trump scenario, because it's a tough act to pull off. Even if Trump were to attempt it, it's more likely he would anger both Republican and Democratic leaders, who would unite to stop his agenda (see scenario 1).

But Trump has proven a surprisingly adept diviner of untapped public sentiment, and it's possible that if he won, a victory would also send such a strong signal that the usual alignments in American politics were outdated that some smart politicians in both parties might adapt to join the Trump wave.

Impressionistic probability of this scenario (conditional on Trump winning): 5 percent

Something else entirely

Obviously, there are other possibilities. If a major war or an economic depression breaks out (still unlikely, but possible), who knows? A President Trump could grab considerable emergency powers and use them in unpredictable ways. The president's powers are greatest in foreign policy, and at times of great crisis.

There's also the possibility we could see a Trump presidency demonstrate more than one of these scenarios at different times. Perhaps he'll start off as a standard Republican but over time will fight with his own party, and perhaps figure out how to be an independent populist who realigns American politics.

The wisdom of checks and balances

For those panicking about a Trump presidency, it's important to keep in mind that the unitary powers of the president are not as great in law as they are in popular imagination. Most likely, the damage Trump could do is pretty limited, because the powers of the presidency are pretty limited.

Again, the most likely scenario is that a President Trump would just be an ineffectual executive, who might ultimately self-destruct in frustration when he finds out just how circumscribed his authority really is.

Liberals who want political change often bemoan the wide-ranging checks and balances and many veto players in our political system, and the way it frustrates legislation. They may be appreciating the wisdom of it very soon.

This post is part of Polyarchy, an independent blog produced by the political reform program at New America, a Washington think tank devoted to developing new ideas and new voices. See more Polyarchy posts here.