Trump has Mulvaney on TV acting chesty and defiant, while gaslighting us on the legal appropriateness of the request and playing I'm-rubber-you're-glue on the question of which party is defying the law. (It's Republicans who know that the text of the law isn't on their side.) Prior to that, Trump sent out one of his personal lawyers, William Consovoy, to argue that the request was an attempt to "harass" Trump because Democrats "dislike his politics and speech." (Yes, Trump is the president of the United States -- but he's being deplatformed! Like Ben Shapiro and Milo!)
This is America, of course, where the federal bench is stocked with extremists placed there by the last two Republican presidents, men who reached the White House through skulduggery and without benefit of a popular-vote mandate. Those judges won't care what the law says.
And if the courts do somehow rule against Trump, all the way up to the Supreme Court, I believe he'll still defy them. I've been saying since 2017 that Trump doesn't have the instincts of a true totalitarian dictator, which we knew when courts ruled against his first Muslim ban and he didn't say, "Screw the courts, we're doing it anyway, exactly the way we planned." Trump still wants to (more or less) color inside the lines, as the other branches of government define those lines. He'll bend the rules massively -- an emergency declaration to get his wall, for instance -- but he'll try to make it seem as if the law is on his side.
I assume that even if he's exhausted all legal channels and all possible strained interpretations of the law, and he's finally compelled to release his taxes to the House, he'll refuse, because while he doesn't have a true totalitarian will to power, he'll do anything to save his own ass, which is what he cares about more than anything else.
As I've said before, if the contents of either the Mueller report or Trump's tax returns are made public, then that's not only the end of the Trump regime, it's the end of the Trump brand. Donald Trump will do anything to prevent that.
And should SCOTUS rule that the Mueller report be released (highly unlikely, we'll never see it) or Trump's taxes (highly likely and almost a certainty to be leaked to the press) then as Steve says, Trump will refuse.
Who will compel Trump to obey the Supreme Court?
Nancy Pelosi and the House Dems? They won't impeach. They'll never impeach.
Mitch McConnell and Senate Republicans? Trump will take every single one of them down with him and all parties know it. They'll never convict him during an impeachment trial.
The American people? They elected him in the first place.
We'll never see Trump's returns.