Republican down-ballot candidates are right now facing the business end of a 60-40 split.
What is a 60-40 split in politics?
If you were to ask a candidate for office, what percentage of voters they would like to agree with their positions, their first intuition would be to say, all of them. But the problem with issues like that is that their opponents will likely agree as well, making the issue politically moot. The issues that elect politicians tend to be ones where a solid majority supports them, but there remains a significant minority on the opposite side.
We are right now seeing an excellent example of this in terms of responding to Trump's candidacy. Roughly 60 percent of Americans are solidifying their views that Donald Trump would not be a successful president, even if a significant chunk of those voters aren't certain that Hillary Clinton would be the best choice either. For many Republican candidates for offices below the presidency, they're now faced with the challenge to either support a presidential candidate who a significant number of the voters in the middle who decide elections strongly dislike, or to abandon their base, without which they have no chance.
This isn't the only such issue that politicians of both parties often face, and generally, they benefit from voters rarely voting according to one issue alone, but rather responding to many. Nonetheless, it's a challenging situation.
One option is to simply grin and bear it. Most Republican Congresspeople have announced that they would vote for Trump, though many have danced on whether this qualifies as an endorsement, and some have tried to avoid ever using his name. Others are going in the opposite direction. We've seen already one Republican Congressperson announce he would vote for Gary Johnson, and a second even say he would vote for Hillary Clinton, but neither of them are running for re-election this year.
A second strategy is to expand the number of issues, and make it not simply a question of who you endorse for the presidency, but how one relates to them. Mike Coffman, a Republican Congressperson from Colorado, who represents a particularly crucial swing-district in a swing-state, released a TV ad in which he said that no matter who the president was, Trump or Clinton, he would oppose them. This is reminiscent of Ted Cruz's convention speech, in which he asked Republicans to vote for any candidate, up and down the ticket, of whom their conscience approved, even, he implicitly suggested, if the presidential candidate didn't so qualify.
On the surface, this is a shrewd strategy. Voters very often vote for one party's candidate for the presidency, and another for Congress; this is how both Dakotas elected only Democratic senators for decades, while routinely giving their electoral votes to Republican presidential candidates, and the state of Iowa used to regularly re-elect one of the most liberal and the most conservative members of the US Senate.
Incumbent Representatives also benefit from the generally positive view that voters have to their own member of Congress, which makes it a little easier for them to convey a subtler response to the controversial person on the top of their ticket.
The problem with this strategy is that there is a very real chance that Republican voters will listen to them and treat this election not as a presidential election, but as a mid-term election. Significantly more voters come to vote when a presidential candidate is on the ticket than when just members of Congress and governorships are up for election.
If Democratic voters treat this election like it's a presidential election, and Republican voters treat this like it's a mid-term election, then the Democrats will win not just the presidency, but all sorts of down-ballot campaigns. Consider Mike Coffman's last two campaigns. In 2012, a presidential year, he won the election with 48 percent of the vote while his Democratic opponent got only 46 percent of the vote (third party candidates got the rest of the votes). In 2014, a mid-term election, his percentage of the vote spiked up to 54 percent. Despite getting a larger percentage of the vote in the mid-term election, however, Coffman got fewer votes then than his opponent in the general election got. Even if Coffman remains just as popular as he was in 2014, if enough would-be Republican voters decide that a Congressional election isn't a good enough reason to even show up to the polls, he'll lose.
It isn't quite clear why so many more voters only show up when a presidential election is on the ballot. One big reason is that voters intuitively understand the importance of the presidency, and are far more likely to simply care more about who is the president. If significant numbers of typically-voting Republicans decide that they don't strongly prefer either candidate, then many might simply not show up. Or, if they do show up, they might not feel it as necessary to insist that all of their friends and family go out to the polls, and might remain silent.
Another possible reason for the spikes in turnout is the role of political organizations. Donald Trump won the primaries without any apparent investment in the data operations needed to identify his voters, or the field staff to encourage them to the polls, and there is little evidence that he has made the necessary investments for the general election. Instead, he has delegated this task to the RNC and the various state parties. Those organizations, however, typically work just as hard for mid-term elections as they do for presidential elections. Typically, the party's candidates benefit from a huge influx of resources by their presidential candidate; Hillary will provide that support to Democratic candidates, but Republican candidates won't get that extra backing.
We often think of Election Day as a single event, but it is when thousands of candidates across the country are each subject to voters who are responding to their own urges. In this election, we might see an enormous number of Republican candidates who are trying to get elected by themselves on their own terms competing with Democrats trying to elect their whole party, down and up the ticket. And that is a very formidable challenge for any candidate, no matter how impressive.