Some time in 2015, Donald Trump made a calculation. If he refused to release his tax returns to the public like every other presidential nominee and president in the past four decades, he’d get criticism from his opponents and the media, but that criticism would be more tolerable than allowing whatever was in those returns to become public. He could take the hit without fundamentally harming his chances of winning and eventually the issue would fade away.

It turned out that he was right — except for the part about the issue fading away. If anything, it’s getting more important, even undermining Republican unity and threatening the party’s chances of passing tax reform.

President Trump on April 16 issued two tweets in which he criticized protesters who marched the day before to demand that he release his tax returns. (Bastien Inzaurralde/The Washington Post)

For that, you have to give liberals some credit. Who would have imagined that they could get tens of thousands of Americans to turn out at protests all over the country to demand the release of Trump’s returns? It’s not what you might consider a sexy issue. Meanwhile, congressional Democrats have signaled that a demand for Trump’s returns will be a centerpiece of their argument against whatever form of tax reform Republicans come up with.

All of this is going to put Republicans in Congress in an uncomfortable position. They desperately want to pass tax reform, or at the very least a big tax cut, even if they don’t do a total overhaul of the system. But the politics of it will already be tricky. Their top tax priorities are cutting individual taxes at the top end and cutting corporate taxes. It just happens that this puts them in a diametrically opposed position to the American public, whose top two complaints about the tax system, according to the Pew Research Center, are that corporations and the wealthy don’t pay their fair share.

Trump’s personal taxes therefore offer Democrats a way to take an abstract and wonky debate and make it more compelling and personal — both for voters and for the news media, which loves to frame stories in terms of personalities, none more so than the president’s. A debate in which every Republican proposal is met with, “How much money is that going to put in Donald Trump’s pocket? We need to know!” is not the one Republicans would prefer to have, particularly when three-quarters of the public says Trump should release his returns.

It’s already having an effect. As the New York Times reported today:

More than a dozen Republicans — from recognizable names like Senator Joni Ernst of Iowa and Representative Mark Sanford of South Carolina to backbenchers like Representatives David Young of Iowa, Matt Gaetz of Florida, Walter B. Jones of North Carolina, Ted Yoho of Florida, Rodney Frelinghuysen of New Jersey and Justin Amash of Michigan — have agreed that Mr. Trump should release his returns.

That’s a modest number at the moment, but that’s only because most Republican members probably haven’t been forced to take a position on the question. But the more Democrats manage to keep it a central part of the tax-reform debate, the more Republicans will have to say where they stand. And while their default reaction is always to get behind their president, in this case his position is practically indefensible. (Note that Sen. Tom Cotton (Ark.) got booed by his constituents for parroting Trump’s utterly bogus line that he can’t release his returns while he’s being audited, which of course he can.)

But it’s one thing to say that Trump should release his returns, while it’s something else entirely to try to force him to do so. While Republicans could theoretically insert a provision into a bill (on taxes or anything else) requiring the president to make his returns public, then pass that bill through both houses and force Trump to veto it, the chances of that happening are close to zero. Your average Republican member of Congress would like to stand up for transparency by saying that he himself believes Trump should release the returns, but that member of Congress would also like the returns never to become public, because they might cause a scandal that would engulf the whole party. So don’t expect Congress to actually force the issue, so long as it remains in Republican hands.

So if your member of Congress says “I believe that the president ought to release his tax returns,” you might want to ask: Okay, what are you going to do about it?

If this remains one of the key points in the argument about tax reform — and if Democrats succeed in branding the eventual Republican bill the “Donald Trump Secret Enrichment Act” or whatever — it raises the political cost of supporting the bill. And Republicans don’t have that much room for defectors in either house. It might not be enough all on its own to kill a tax cut, but the more controversial the bill becomes, the higher the cost of supporting it becomes.

That’s not all. If the Democrats can take back the House in 2018, which looks like a possibility, we’re probably going to see Trump’s returns.

Why? Two words: subpoena power. If Democrats controlled the House, their representatives would chair all the chamber’s committees, and multiple committee chairs would very quickly find reason to demand Trump’s returns. In February, Democratic Rep. Bill Pascrell Jr. (N.J.) noted that a 1924 law allows the chair of the Ways and Means Committee to request anyone’s tax returns from the IRS, including the president’s. The committee could then distribute the returns to the entire House, which would effectively make them public.

And that’s just one committee. If Democrats took over, the Government Oversight Committee would presumably be led by Elijah E. Cummings (Md.), who has spent the past few years feuding with the Republican chairmen of that committee, first Darrell Issa (Calif.) and now Jason Chaffetz (Utah). I’m pretty sure that if Cummings put his mind to it, on his first day as chairman, he could come up with 10 or 20 different rationales for why his committee needed to see Trump’s returns. Every Democrat would want to get into the act — don’t be surprised if the chair of the Subcommittee on Industrial Adhesives and Post-Nasal Drip decided there was an urgent need to see those returns pursuant to their investigation into, well, whatever.

If and when those subpoenas arrive, Trump would sue to quash them. It would ultimately be decided by the Supreme Court, and he might win. But he might not. And then the real investigation would begin.