Republicans need to start worrying about losing their majority in the House of Representatives.

Republicans accept the conventional wisdom that Hillary Clinton is favored to win the presidency, and they know that her election would probably end their majority in the Senate. But in a year that has upended political expectations, they have clung to one comforting assumption: Their hold on the House is secure.

Their majority is protected by gerrymandering, the geographic distribution of Republican voters, the power of incumbency and its own sheer size. Republicans have 247 seats in the House, the most since 1931. Democrats would have to win 30 to take back the chamber. And that includes many seats in districts that usually go Republican in presidential contests. That sets the House apart from the Senate, where to keep their majority Republicans will have to hold seats in states that usually vote for Democratic presidential candidates.

But Clinton’s lead in the polls is widening to the point that Republicans need to set aside their complacency. Split-ticket voting has declined over the last generation. If Clinton wins big — because Republican voters stay home, or swing voters choose her party, or both — House Republicans will struggle to win re-election. Henry Olsen, the co-author of a recent book about the Republican Party, tells me that an eight-point win would put Republicans in the danger zone.

Now it may be that the decline of split-ticket voting is another pattern this year will disrupt. Maybe voters will distinguish between Donald Trump and other Republicans, rejecting him but not them. Maybe Republican voters will come to the polls to vote for a third-party presidential candidate, such as Libertarian Gary Johnson, and then vote for Republican congressmen while they are there.

Public polling on the congressional races is still sparse. The most recent numbers come from Ipsos/Reuters, which found the Democrats with an 11-point lead nationally. That could be a sign that Trump is pulling Republican congressional candidates down with him.

Republicans should also consider that their optimistic take on the House sounds a lot like what they said in 2005 and 2006 as their political fortunes declined during President George W. Bush’s second term. Then, too, the district lines and voters’ preference for incumbents were supposed to keep Republicans in control of Congress. In late May of 2006, the respected political analyst Stu Rothenberg projected that Democrats were likely to gain 8 to 12 seats, leaving Republicans in charge of the House. They ended up winning 31 seats, picking up the House and further surpassing expectations by taking control of the Senate, too.

That history is not repeating line for line. A decade ago the Democrats were very successful in getting qualified candidates who were good fits for their districts. Recruitment does not seem to have gone as well this year.

If the presidential race goes well enough for the Democrats, though, it might not matter. “If worse comes to worst, I think the House is more at risk than people realize,” says one prominent Republican strategist who requested anonymity so that he could offer a candid assessment. He says that if Trump looks like a sure loser, congressional Republican candidates should do what they did in 1996, when their presidential nominee Bob Dole seemed doomed to defeat: Present themselves as necessary checks on a Democratic president named Clinton.

House Republicans are more likely to retain their majority than Trump is to win, and for that matter a Trump victory can’t be ruled out. But Republicans should not kid themselves about the worsening political environment. A year ago they thought that they could begin 2017 with control of the White House, Senate, and House. Now they have a real prospect of being shut out of national power altogether.

E-mail Bloomberg View columnist Ramesh Ponnuru at rponnuru@bloomberg.net.

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