Democrats and their narrow majority in the House will be an issue for Republicans to exploit

Steve Berman:

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The vaunted "blue wave" never really materialized, with Democrats narrowly picking up the House of Representatives it was widely believed they would win anyway. With 22 close races left uncalled, Decision Desk HQ has the Dems up 221, with only 215 required for a majority. A narrow majority in the House isn't enough to do much without also holding the Senate and the White House, and Democrats know it.

And historically, the House usually flips (it has every time the White House changed parties except twice in the last 100 years).
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The Democrats not only didn't capture the Senate, they lost ground. They lost their headline premier races: Beto, Abrams, Gillum. There is no credible way for the liberal press to declare victory for Democrats, though they huff and they puff and they try to blow the Trump House down.
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The House of Representatives is ephemeral and changes easily every two years. Such a narrow majority will prove to make Nancy Pelosi and company regret it.

Trump will become a flamethrower, and she will be his favorite target. The GOP will campaign off Pelosi for two years, raising money and building a war chest. I can already see Republican consultants and media buyers licking their chops. Of course, that doesn't guarantee Trump victory in 2020. Far from it, there are some serious canaries choking in coal mines.
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"There's only been 5 times in the last 105 years that an incumbent President has won seats in the Senate in the off year election. Mr. Trump has magic about him," Trump tweeted Ben Stein's words.
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If the Democrats use their narrow majority to attack Trump rather than try to govern they will be on the receiving end of much unpleasantness.  Ask Jon Testor who is currently losing his Senate race in Montana.