The second is that it gives Trump a boogeyman — or, more aptly, a boogeywoman. It’s one thing to campaign against House Minority Leader Nancy Pelosi (Calif.) by warning that she could become speaker; it’s another to do so if she is speaker. If Democrats win the House, Trump will have a ready-made foil for his 2020 reelection campaign, even if his Democratic opponent is not such a lightning rod.
Trump could also blame the Democratic House for his continued failures to live up to his many, many promises. (He has already done this to some degree, even though Democrats have no control over any branch of government.) Trump has fulfilled some promises, but key ones and far-fetched ones such as the border wall (not to mention making Mexico pay for it) remain improbable even if Republicans keep control of the House and Senate. If you are going to have gridlock, you might as well have someone to blame it on who is not in your own party.
And finally, even that subpoena power could pose some tough choices for Democrats. There will be pressure from the party’s base to go after Trump heard and even impeach him, but we’ve seen how that can lead to overreach — most notably, when Republicans impeached Bill Clinton in the late 1990s. And Democratic leaders have already telegraphed a wariness about that. What happens when they actually have power and the base wants them to go further than they think is prudent? That doesn’t mean they will go too far, but there’s a clear tension.