Democrats are feeling cheery at the prospect that this fall will bring an end to the power outage they suffered in 2016 and the near-irrelevance they have endured since.

Conditions look particularly good for taking back the House. Off-year elections in a president's first term nearly always cost his party some seats, and President Donald Trump's historically low approval ratings are an especially heavy weight for Republicans.

Democrats also have enthusiasm on their side, something that hasn't been the case in a nonpresidential year since 2006. In the latest Post-ABC News poll, more than half of Democratic-leaning voters said it has become "more important to vote" this year; only a third of Republicans felt that way.

So, what could go wrong for Democrats in 2018?

Plenty, actually.

Already, Democratic strategists are getting a little jumpy about the party's shrinking advantage in the polls, especially the closely watched generic-ballot test, where voters are asked which party they would prefer to represent them in Congress.

The spread is running at 6.5 percentage points in the RealClearPolitics average -- just half where it was at the beginning of the year. It shows an electorate still inclined to vote Democratic but probably not by enough to flip the House.

What should be more worrisome to Democrats -- ironically enough -- are some of the very forces that are working in their favor.

They are on track to have a record number of candidates running, led by a surge of women and veterans. According to the latest compilation by the Campaign Finance Institute, nearly 60 percent of Republican-held districts have a Democratic challenger who has raised at least $50,000; two years ago, fewer than 20 percent had reached that threshold by this point in the cycle.

All of that is a good thing for any party. Except when it leads to large, messy primaries.

In crowded primaries, Republicans will be rooting from the sidelines for the liberal base to pull all of the candidates leftward -- forcing them to declare allegiance to single-payer health care, impeaching Trump, free college tuition. Those are October attack ads in waiting in many of the scattered pockets where Democrats need to win.

Meanwhile, economic confidence has reached a level not seen in 17 years. That suggests, among other things, that the tax cuts that looked so unpopular when they were passed in December may be an asset to Republicans by fall.

Democrats also have to contend with two givens in Republicans' favor -- gerrymandered districts and outside spending by conservative groups.

As district lines are now drawn, a disproportionate amount of Democratic energy is corralled into a relatively small number of congressional seats. On the money front, Democrats were already expecting to be outspent by at least $100 million. Then the Koch brothers informed their donors late last month that they were going to put as much as $400 million into the midterms, their largest commitment ever. In House races, a million dollars here and there can make the difference.

Finally, perhaps the single biggest miscalculation that Democrats could make right now is to expect Trump to do all their work for them.

One thing to remember about waves. Most of them break before they reach the shore.

Karen Tumulty, a new Washington Post Opinions columnist, has been a Post national political correspondent since 2010.