It turns out that, with apologies to Nancy Pelosi, Republicans really did have to pass the tax bill so people could find out what’s in it.

The GOP has made gains on the generic congressional ballot in recent weeks, with warmer feelings about the tax plan contributing to the upward trend.

The improving numbers at least raise the prospect that, just as in 2016, Democrats will be lured by their abiding conviction in President Trump’s inevitable failure and their deep loathing of him to misplay what should be a winning hand.

There’s no doubt that Republicans are in trouble. Special elections, fundraising, retirements and history all suggest it will be a strong Democratic year.

A party with control of both elected branches in Washington and possessing just a 24-seat House majority would be in danger of losing control of Congress even with a pleasingly anodyne president, tweeting lines from Hallmark cards, at the helm.

But the new polling is alarming enough for the left that the progressive group Priorities USA released a memo suggesting a readjustment. It posits that Democrats were winning the tax and health-care debates, then “in the last few weeks, Democrats turned their attention to other issues.”

What other issues? Well, every other issue. Immigration, of course. But, otherwise, the crisis du jour — “s – – thole countries,” etc. — and the hardy perennials of Russia and obstruction, sometimes switched around to obstruction and Russia for novelty’s sake.

Trump is the racetrack rabbit that keeps Democrats running in a perpetual cycle of outrage. This doesn’t mean the president is playing three-dimensional chess, or that the unending Twitter-fed controversies help him.

Surely, if he arranged for a 60-ton M1 Abrams tank to ceremonially crush his phone at his hoped-for military parade down Pennsylvania Avenue, his approval rating would bump up a couple of points.

But how many voters care about whatever the latest flare-up is as much as the professional commentators and partisans whose job it is to get exercised by the controversy of the hour?

In their perpetual state of high dudgeon, Democrats are inviting voters to fixate on the uproars, while telling them to dismiss bonuses and pay increases — “crumbs,” as House Minority Leader Pelosi deemed them — that are much more tangible.

Per the Priorities USA polling, Trump still has dreadful ratings on anything having to do with how he conducts himself, but he is making progress on bread-and-butter issues.

The latest Priorities USA survey has him at 34 percent favorable to 46 percent unfavorable on health care, up from a dismal 23 percent to 53 percent last November; 46-42 on taxes, an increase from 32-48; and 46-39 on the economy, a marked improvement from 38-43.

The backdrop here is the tax law. The ObamaCare bounce that never materialized for Democrats — at least not until Republicans began trying to repeal the health-care law — has shown up for the GOP on taxes.

It would be difficult to craft a proposed tax cut with worse polling before the bill passed. The numbers were worthy of a tax increase — and, in fact, were largely based on an erroneous belief that the bill indeed constituted a tax hike. A solid plurality expected their taxes to increase, against the evidence of every credible evaluation of the bill.

Almost immediately after its passage, companies began giving their employees bonuses and pay raises. Now, the cuts on the individual side are kicking in. Quinnipiac and Monmouth have the public evenly split on the law, and there’s clearly room for more growth.

Priorities USA is right to urge Democrats to take heed. The memo doesn’t put it in these terms, but it argues that the key to defeating Trump is normalizing him, running against him as a conventional Republican crony of the rich and big business who is redistributing income upward. This Democratic message has worked pretty effectively for 80 years — why change it now?

But a party that’s in a perpetual state of hysteria and, on top of that, is operating against the backdrop of an economy with 4.1 percent unemployment may find it harder than expected to pull off its old tricks. The current interlude may not mean Republicans escape their hanging in the fall, but it gives them something in short supply before passage of the tax bill — hope.

Twitter: @RichLowry