A look inside the complex relationship between Hillary Clinton and Barack Obama, 2 of the most powerful people on the planet

Jim Bourg/Reuters

President Barack Obama endorsed Hillary Clinton in a Thursday afternoon video posted to the presumptive Democratic nominee's YouTube account.

"I know how hard this job can be. That's why I know Hillary will be so good at it. I don't think anyone has been more qualified for this office," Obama said.

Clinton welcomed Obama's endorsement, saying it "means the world" to her that her former rival has her back in the 2016 campaign, Clinton told Reuters.

"It is absolutely a joy and an honor that President Obama and I, over the years, have gone from fierce competitors to true friends," Clinton said.

President Obama will begin campaigning with Clinton next week in Green Bay, Wisconsin.

 Below are 23 photos of two of the most powerful people in American politics.

Barack Obama and Hillary Clinton laugh near the end of the CNN/Los Angeles Times Democratic presidential debate in Hollywood, California.

Then US Democratic presidential candidates Barack Obama and Hillary Clinton pose for photographers prior to the CNN/Los Angeles Times Democratic presidential debate in Hollywood, California January 31, 2008.

Then US Democratic presidential candidate Barack Obama listens to Hillary Clinton on a plane after leaving Washington June 27, 2008.