A majority of Republicans now support increasing taxes on wealthy Americans, according to a new poll — an abrupt shift against a long-held tenet of GOP economic doctrine.

The results from the Public Religion Research Institute and the Brookings Institution show that 54 percent of Republicans support increasing taxes on those with incomes over $250,000 a year, an increase of 18 percentage points since the last presidential election in 2012. Among Americans as a whole, 69 percent support an increase.

The figures suggest that the tax reforms proposed by Donald Trump, Paul Ryan and other prominent Republicans — which would reduce taxes for the wealthy — are at odds with their constituents' wishes.

The Public Religion Research Institute's polling indicates a sudden change in Republicans' attitudes on taxing the wealthy just in the past couple of months. To a lesser extent, there was also a shift in attitudes among Democrats and independent respondents. In the most recent poll, 68 percent of independents and 84 percent of Democrats supported an increase in taxes on the wealthy.

It is too soon to say whether the results represent a real shift in opinions or are just some kind of statistical illusion, said Dan Cox, the institute's research director. While there was no change in the design of the poll or the wording of the question that could account for such a large change, Cox plans future polls to answer the question conclusively.

"You always have to ask yourself that question: is this an artifact of the methodology?" he said. "Or could it be something about the moment that we’re in right now?"

At least for now, though, the data suggests that Republican voters are giving up on the idea that reducing the rates paid by the wealthiest will encourage them to work more and invest more, stimulating the economy and making everyone else better off.

"We've always overestimated the extent to which Republicans were completely on board with the sort of go-go supply-side economics of the 1980s," said James Pethokoukis, a commentator on taxes and economic policy at the conservative American Enterprise Institute.

It could be that those old-fashioned trickle-down theories are less appealing today. Eight years after the financial crisis, frustration is mounting for many as salaries and wages have improved little.

"To some extent, the tide has risen for everybody," Pethokoukis said. "Obviously, it’s risen a lot faster for people at the top, and it’s hard not to take notice of that."

Increasing taxes on those earning more than $250,000 a year was one of Obama's proposals during his campaign for reelection in 2012. Ultimately, he reached a compromise with congressional Republicans, allowing President Bush's reductions to expire for individuals with more than $400,000 a year and couples with at least $450,000.

The results of the poll could also affect Trump's strategy approaching the general election. The plan he published last fall offered vast tax relief to the wealthy, but he has also said that some rich taxpayers should pay more, and he recently indicated that he is reconsidering the plan he had initially put forward.