GOP Sen. Toomey banking on split-ticket voting in Pa. race

Republicans note that Trump became known to the public not as a member of their party, but as a mogul and reality TV star, which might help voters differentiate him from down-ballot GOP candidates.

"Right now, if you look at the way these races are coming together, I think that the Republican candidates by and large are significantly over-performing where you would expect them to be, given the state of the presidential race," said Josh Holmes, a GOP consultant and former chief of staff to Senate Majority Leader Mitch McConnell, R-Ky.

If Trump continues to lose ground, some Republican candidates may begin to argue outright that voters need to elect Republicans as a check on a Clinton administration, a strategy used in 1996 when Republican presidential nominee Bob Dole was on track for a loss.

"By the time we get to November, I think it's a really hard argument to make to people that 'I hope you'll just ignore our presidential race and come out and vote for me,'" said Martha McKenna, a Democratic media consultant and former political director with the Democratic Senatorial Campaign Committee.