GOP tax reform used to be unpopular. Not anymore.

When the Republican-controlled Congress first approved its tax bill in December, most Democrats believed it would be a political loser for the GOP. Indeed, a New York Times poll found that just 37 percent of Americans approved of the plan. “To pass a bill of tax cuts and have it be so unpopular with the American people is an amazing achievement for the Republicans — it’s never been done before,” Senate Minority Leader Charles E. Schumer, D-N.Y., crowed.

He’s not crowing anymore. In January, support had risen to 46 percent, and this week it reached a 51 percent majority. Meanwhile, disapproval has dropped from 57 percent in December to 45 percent today. That is a swing of 26 points.

Why the change? Because taxes are personal. The tax bill is complicated, and when it was passed, many Americans were confused about how it would affect them. A December poll found that only 17 percent believed they would pay less in taxes, while 32 percent thought they would pay more. In fact, about 80 percent of taxpayers will receive a tax cut this year averaging about $2,100, according to the nonpartisan Tax Policy Center.

It is taking time for voters to figure this out. In 2001, when President George W. Bush passed his across-the-board tax cuts, his message was simple: Every American who pays taxes will get a tax cut. But in 2017, Republicans overhauled more of the tax code. They cut tax rates and doubled the standard deduction but also eliminated many traditional deductions for those who itemize. As a result, a lot of taxpayers didn’t know whether they would end up winners or losers.

Now, more Americans are starting to discover that they are winners. Millions are starting to receive their Trump tax cuts as employers lower their tax withholdings, leaving more money in their paychecks. And the bill will become more popular as more people learn the good news. Even now, only one-third of Americans think they will see an income tax cut. Many voters are going to be pleasantly surprised when they discover their taxes are being reduced thanks to President Donald Trump and Republican lawmakers. And that does not take into account the bonuses and raises that many Americans are receiving from their employers because of the corporate tax cuts.

When House Minority Leader Nancy Pelosi, D-Calif., dismissed these tax saving as “crumbs,” she came off as an elitist. A couple of thousand dollars a year may be crumbs to a San Francisco multimillionaire, but to most hard-working Americans that is real money. Indeed, Pelosi is getting slammed by her fellow Democrats for her out-of-touch response.

Democrats are in a bind because they voted against that increase in take-home pay. The growing public support for tax reform is especially a problem for the five vulnerable Senate Democrats running for re-election in states that Trump won by double digits.

The success is already bolstering GOP candidates. In North Dakota, a state Trump won by 36 points, the success of the tax bill has drawn Democratic Sen. Heidi Heitkamp a strong challenger in Republican Rep. Kevin Cramer. Just last month, Cramer had announced that he was not going to run. Why did he change his mind? The Post reports that one reason was Cramer’s belief that Heitkamp’s vote against the tax bill was “fatal” for her.

Expect to see a lot of other GOP challengers — and purple-state voters — come to the same conclusion.