Right now, most bettors foresee nine words that are the stuff of Republicans’ 2017 nightmares: “President Hillary Clinton and Senate Majority Leader Chuck Schumer.” But at least the House is safe for the GOP. It is, right?

House Democrats probably need a Donald Trump loss of historic proportions to have any chance at a three-part sweep. But not even a Clinton rout would guarantee that scenario thanks to structural factors and because voters skeptical of both nominees could well anticipate such an outcome and respond to a Republican message of “checks and balances” — a tactic that’s worked before.

Republicans hold their largest House majority — 247 seats to 188 for Democrats — since the 1928 election, in part because they have some tremendous built-in geographical advantages, both natural and engineered, that their counterparts in the Senate don’t share.

First, Democratic voters have never been more concentrated in big urban areas than they are now. In 2012, President Obama won by 126 electoral votes while carrying just 22 percent of America’s counties — even fewer than losing Democratic nominee Michael Dukakis’s 26 percent in 1988. That means Democrats are wasting more votes than ever in safe congressional districts they already hold. For example, an additional straight Democratic ballot cast in Chicago or Madison might help defeat GOP Sens. Mark Kirk in Illinois or Ron Johnson in Wisconsin, but it’ll do zip to put a dent in Speaker Paul Ryan’s House majority, because Democrats already hold all the House seats anchored by those cities.

Second, Republicans’ astounding state legislative gains in the 2010 midterms — the year before the decennial redistricting cycle — allowed them to redraw four times as many congressional districts as Democrats in 2011 and 2012, stretching their geographical edge even further. As a result, in 2012, Democrats won 51 percent of all major-party votes cast for House candidates but just 47 percent of all seats. In 2014, Democrats won 47 percent of all major-party votes but just 43 percent of the seats. Amazingly, just 16 of 247 House Republicans won their races by fewer than 10 percentage points.

If Democrats’ seat share continues to lag their national vote share by about 4 percentage points in 2016, the party might need to win about 8 percent more votes than Republicans nationally just to reach the barest possible majority of 218 seats.

It’s very possible Clinton could achieve that margin of victory. But even in an era of high straight-ticket voting, extending those coattails to House races will be difficult because Republicans’ down-ballot advantages still exceed Democrats’. Let’s take a quick tour:

Why Democrats will gain House seats in 2016:

Rebound — Quite simply, Democrats hit rock bottom in 2014, when their base, especially young and nonwhite voters, didn’t show up. It cost them some districts the GOP has no business holding. As a result, Democrats are likely to bounce back in several districts they lost, particularly against fluke winners such as GOP Reps. Rod Blum (IA-01), Cresent Hardy (NV-04) and Will Hurd (TX-23).

Court-ordered redistricting — Democrats rejoiced when courts threw out Republican-drawn congressional maps in Florida, North Carolina and Virginia on grounds of partisan and racial gerrymandering. However, the net effect of new court-ordered maps is likely to be small: Democrats will probably pick up one seat each in Florida and Virginia, for just two seats total out of the 30 they would need for a majority.

Open seats — There are a dozen more House Republicans retiring or running for other office this year than Democrats, 30 to 18. Of those, according to the Cook Political Report’s ratings, 10 GOP seats are vulnerable to Democratic takeover and just four Democratic seats are vulnerable to GOP takeover. Historically, open seats have been among the most sensitive to national vicissitudes — such as, potentially, a Trump bust.

Why Republicans will probably keep their majority:

The great sort — The House is just a lot less “elastic” than it used to be. Today, the Cook Report rates just 36 of 435 districts as competitive — about 8 percent of the House. Even if Democrats were to hold all their own seats and sweep out all 26 Republicans sitting in districts Obama carried in 2012, they’d still be four seats short of a majority (and, by our tally, just five of these 26 Republicans have endorsed Trump by name so far). By contrast, if Democrats were to defeat all seven Republicans running for Senate in Obama states, they would win a 53-seat majority, assuming they hold all their own seats.

Timing — It’s possible there are a lot of talented Democrats who would be running for Congress right now if only they had known Trump would be the GOP’s presidential nominee. The problem is, by the time Trump effectively wrapped up the nomination on May 3, the filing deadlines to run for Congress had already passed in 79 percent of districts. As a result, Democrats are stranded without top-tier candidates in several GOP seats where Trump could lose badly, such as NJ-03 and PA-06, both located in the Philadelphia suburbs.

Ticket-splitting — Although there isn’t a ton of publicly available polling at the House level yet, some Democratic operatives say that for down-ballot purposes, they’d rather be running against Ted Cruz than Trump. Why? Whereas many voters see Trump as his own (radioactive) island and an entity separate from politics altogether, Democrats could have used Cruz’s voting record as a sitting senator to effectively tie House GOP incumbents to an unpopular presidential nominee.

At the moment, the likeliest outcome seems like a Democratic gain of five to 20 seats (the Cook Political Report rates 22 GOP-held seats as Toss Up, Lean Democratic or Likely Democratic, compared with four Democratic seats in Toss Up, Lean Republican and Likely Republican). In other words, the first few GOP targets are very winnable for Democrats, but the last few needed for a majority would require a wave.

However, don’t underestimate the effect that Democrats cutting the GOP majority in half might have. It could have big consequences for governing. Back in October, we predicted that Paul Ryan wouldn’t have it any easier than John Boehner did when it comes to fundamental spending and debt votes, thanks to rebellions from the very conservative House Freedom Caucus.

If Ryan were to lose half his 30-seat majority, he could be the last backstop against a Democratic White House and Senate. But Ryan would also likely be forced to reach across the aisle for Democratic votes even more often than Boehner did, giving the minority more leverage and possibly branding him as the GOP’s RINO-in-chief for good.

For reasons beyond simply the Trump conundrum, the speakership is looking less and less like the job Ryan signed up for eight months ago.