The U.S. Capitol building. (Joshua Roberts/Bloomberg)

James Downie is The Post’s digital opinions editor.

It’s all coming up aces for the Democrats. Hillary Clinton leads Republican Donald Trump by more than seven percentage points nationally in the poll averages. She is ahead by seven points in New Hampshire, eight in Virginia, nine in Pennsylvania and 11 in Colorado — enough to lock up the electoral college. Polls also look good for the Democrats to retake the Senate, especially because former Indiana senator Evan Bayh decided to run for his old seat.

But not all the news is good. Despite the Democrats having the inside track for the executive branch and the upper chamber of Congress, there seems little chance of a Democratic House.

Why? The easy answer is that the Democratic Party — and especially the Democratic Congressional Campaign Committee — has failed to take advantage of its opportunity. With the White House nearly clinched and the Senate ripe for retaking, the argument goes, surely the only reason a House takeover appears remote is institutional failure. While the DCCC would like to make between 45 and 60 seats competitive, outside prognosticators put the number in the mid-30s.

There is no doubt that the DCCC and the party could have done a better job preparing for a Trump-led ticket. Though Trump’s nomination helped the DCCC recruit, in many cases his victory came too late: By the time Trump essentially clinched the nomination in March, three-quarters of the filing deadlines had passed. Frankly, though, staffers at places like the DCCC are paid to anticipate things like a Trump victory. “There are a dozen districts where Democrats should have been able to compete with Trump at the top of the ticket,” says David Wasserman of the Cook Political Report.

Democratic presidential nominee Hillary Clinton leads her Republican opponent Donald Trump by 8 points in a new Washington Post/ABC News poll. The poll also found a majority of voters see both candidates as dishonest. (Bastien Inzaurralde/The Washington Post)

A dozen districts, in other words, that might have been in play but can be counted as safe by Republicans. Democrats have a good roster of recruits in the most “flippable” seats, from rising stars such as Morgan Carroll in Colorado’s 6th District to veterans such as Carol Shea-Porter in New Hampshire’s 1st. But Mike Bost in Illinois’s 12th District and Tom MacArthur in New Jersey’s 3rd are just two of a number of GOP members of Congress who can breathe more easily because Democrats could not find strong opponents for them. And though Darrell Issa won his intraparty primary only 51 percent to 45 percent in an increasingly Hispanic (and therefore anti-Trump) district, the DCCC seems reluctant to help dark-horse opponent Doug Applegate capi­tal­ize. No doubt Issa’s large personal fortune scares Democrats, but laying the groundwork for a national wave requires making some long-shot races competitive.

All is not lost for the Democrats. Trump’s nomination gives House candidates something they can use to convince donors that normally tough districts are competitive, netting them badly needed funds before Election Day. If a Democratic wave is truly a possibility, one sign will be that House races that right now look safe for the Republicans will start to tighten in the coming weeks, as voters tune in and GOP candidates struggle to defend Trump. Just this week, Larry J. Sabato’s Crystal Ball noted that Republicans might have to play defense in Indiana’s 9th District and Kansas’s 3rd — two seats previously considered safe for them.

Yes, the DCCC may deserve some blame for Democrats’ struggle to flip the House. But the sad truth is that, as Wasserman says, “The Democrats’ best chance of taking back the House is a Trump presidency.” Without the added fuel of a backlash to an incumbent GOP administration, winning the House will be nearly impossible. With Trump both trailing badly and embarrassing many key conservative donors such as the Koch brothers, millions of dollars that would have gone to support a different GOP presidential nominee will now go to defending the House and Senate majorities. Furthermore, gerrymandering, the coalescing of Democrats into high-density urban districts and the decline of split-ticket voting have all made it more difficult for either party to flip a large number of seats.

Clinton and the Democrats will no doubt celebrate on Election Day. But the subsequent elections will be the true tests. Even if the Democrats retake the Senate, the GOP House will surely block every major piece of legislation the new administration seeks, and then blame Clinton for the gridlock. The 2018 midterms, with lower turnout and 23 Democratic Senate seats up for grabs (vs. only eight Republican seats), will be difficult for liberals. If they ever hope to truly advance a progressive agenda, Democrats need to start working now on surviving those midterms, retaking state legislatures and reversing gerrymandering — or Republicans’ stranglehold on government will last even longer.