For now, the sources of potential bias will tend to increase the likelihood that the polls overstate the standing of Republicans. But this is by no means assured.

The polls will usually be fairly accurate, given the various ways pollsters are trying to respond to the problems. They might be more likely to be accurate in a midterm year like 2014, when just about every source for bias seems less likely to be a factor. The young, mobile, new and Hispanic voters missed by telephone surveys are much less likely to vote in midterm than presidential elections. This year, there are also relatively few Hispanic voters in the Senate battleground states; only Colorado has a modest-size Hispanic population.

It certainly remains possible that Democratic mobilization efforts will confound likely voter screens by bringing hundreds of thousands of irregular voters to the polls. But it is also possible that likely voter screens will permit irregular Democratic voters who say they will vote when asked, but do not usually vote in midterm elections and may not ultimately do so. This appeared to happen in Virginia’s 2013 governor’s race, when polls showed the Democrat, Terry McAuliffe, leading by 6 points in a race he would ultimately win by 2.5 points. Early voters will also represent a substantial share of the vote in every battleground state, with the exceptions of New Hampshire and Kentucky — perhaps the two least competitive of this year’s battlegrounds. A larger proportion of early voters reduces the importance of likely voter screens, since pollsters will count every respondent who reports having already voted.

The biggest reason to be skeptical that the Democrats will fare better than the polls predict is the context: The Republicans probably have a large enough advantage to withstand another round of modest polling errors. Even if there is another three- or four-point error in Colorado, for instance, the result would be a dead heat in a race that on its own is not at all sufficient for Democrats to hold the Senate. And the Republicans could just as easily counter the effect of any polling error by winning undecided voters, who tend to disapprove of Mr. Obama’s performance — along with their incumbent Democratic senator, Mark Udall.

Even with the problems that polls have, the Republicans’ advantage is clear enough to make them favorites to win the Senate. But if we wake up to the prospect of a Democratic Senate on the morning after Election Day, we should look to the challenges of modern polling as a big reason for the surprise.