“I think it’s selling itself,” President Donald Trump said, with customary bravado, as he signed into law the $1.5 trillion, GOP-passed tax cut. Every Republican running in a competitive race in 2018 is hoping he’s correct, but they face an uphill climb to turn the president’s words into reality.

There is a sizable disconnect in public opinion shaping the political environment. Economic and consumer confidence are strong, but that has yet to give lift to the president or the GOP. Trump’s approval ratings hover at or just below 40 percent at a time of low unemployment, a booming stock market and steady ­economic growth.

When such a pattern of low presidential approval at a time of low unemployment has appeared in the past half-century, the party in power suffered major setbacks in the midterm elections. Republicans hope the tax measure provides the elixir to produce a change in fortune, but there are several reasons to be skeptical.

The first is the degree to which Republicans failed to sell the tax bill before they pushed it through the House and Senate with record speed. To the extent that the measure is selling itself, as the president said, it is through the responses of a number of corporations that have announced hikes in the minimum wage, bonuses for their workers or new investments.

If these examples are a leading indicator, then Trump could be proven correct in claiming that the tax bill will trigger the kind of widespread response by the business community that ultimately will produce enough investment to create jobs in big numbers and increases in wages for working Americans after a lengthy period of wage stagflation.

But is corporate leaders’ enthusiasm about the measure surprising? It cuts the corporate tax rate from 35 percent to 21 percent and provides other tax breaks designed to spur activity; those cuts are the heart of the measure. But the overall response from the business community and the ultimate effect on the economy remain in question. The forecasts are more modest than the president’s rhetoric.

That’s not the only reason Republicans should be cautious. Because the benefits are heavily weighted toward corporations and the wealthiest Americans, the public has already made a judgment: They feel left behind. An NBC News-Wall Street Journal poll, released as the bill was nearing final passage, found that 63 percent of the public said the bill favors the rich and corporations. Only 7 percent said the measure was designed to mostly help the middle class.

Republicans say the middle-class cuts are more substantial than many Americans know and that taxpayers will begin to change their assessment early next year, when they see more dollars in their paychecks due to the near-doubling of the standard deduction and an increase in the child tax credit. Still, it could take until tax filing in April 2019, well after the midterm elections, before people can fully measure the impact of those changes, along with the effects of the reductions in the deductibility of state and local taxes and for some the lower cap on mortgage interest deductions.

Democrats, with the Affordable Care Act, found how difficult it can be to change initial impressions of a big policy change enacted along partisan lines. (And that measure was judged more favorably from the start than the Republican tax bill.) The ACA proved an anchor around the necks of Democrats in the 2010 election, which ended up as one of worst drubbings for a party in power in generations.

“That’s a good demonstration that it’s difficult to take a bill that’s underwater and change public perceptions,” said Bill McInturff, a Republican pollster and part of the bipartisan duo that conducts the NBC-Wall Street Journal surveys. “We have to acknowledge that.”

But McInturff said the tax measure could be a chip on the table for Republicans in the midterm sweepstakes. If 2018 is an even stronger year economically, he said, there is an opportunity for Republicans to connect that performance to the tax bill and claim credit.

McInturff hopes that corporations will step forward in the coming months with many more examples of how they are taking advantage of the bill’s provisions to expand and benefit their workers — and that the president uses those examples to campaign vigorously highlighting them.

But individual examples don’t paint the whole picture, nor are they always what they seem at the time they are announced. The proof ultimately will be in the overall performance of the economy.

The other, likelier bigger problem for Republicans, however, is the disconnect between perceptions of the economy and the president’s personal ratings — and the degree to which the midterm elections turn on attitudes about Trump rather than personal finances and the economy.

That same NBC-Wall Street Journal poll found that by a margin of 40 to 21 percent, Americans say Trump has helped to make the economy better. Yet that has done nothing so far for the Republican Party. The party’s image is poor and somewhat worse than the Democrats and, as the survey also showed, the GOP is surrendering its advantage on the economy and taxes.

This is perhaps related to initial evaluations of the tax bill and to the fact that on three other measures — America’s standing in the world and two dealing with partisan divisions — majorities say the president has made things worse. In other words, judgments about Trump’s performance and personality remain demonstrably negative, and those attitudes could overshadow the importance of the economy as voters make choices next year.

That’s the conundrum for Republicans as they look to next year. The new tax law is the achievement they needed to construct a case to take to the voters. But midterm elections usually don’t turn on positive messaging; instead, the most motivated voters in midterm elections tend to be those with grievances.