The expected passage of a tax-cut overhaul Wednesday would give President Trump the first major legislative victory of his term. (Jabin Botsford/The Washington Post)

President Trump is poised to achieve what has eluded him throughout his tumultuous first year in office — a major legislative victory.

With the expected passage in Congress on Wednesday of a tax-code overhaul that also dismantles part of the Affordable Care Act, Trump hopes to shed the label of legislative loser. But whether the probable win could help him pass other agenda items in 2018 and catapult him onto firmer political ground ahead of the midterm election is an open question.

Trump’s tax-cuts achievement could be compromised by the unpopularity not only of the legislation itself but also of his own performance in office. Recent polls show Trump’s approval ratings reaching new lows, with a clear majority of Americans holding negative views of his presidency.

And the hyperpartisan divide on Capitol Hill — where the Republican majority in the Senate will drop by one to just 51 out of 100 seats with the arrival of Democrat Doug Jones of Alabama, who defeated Trump-backed Republican candidate Roy Moore — makes it exceedingly difficult for Trump to push other legislative priorities in an election year, such as new infrastructure spending.

Still, many Republicans see the tax plan both as the fulfillment of a decades-long promise as well as one of the first signs that Trump’s agenda can overlap with their own.

Tennessee Gov. Bill Haslam (R) said that this is a milestone for Trump to show voters he can govern and that electing Republican congressional majorities can yield results. All year, Haslam said, Republican voters have been asking, "What's the purpose of Republicans having these majorities and the White House if they can't get anything done?"

“Passing the tax-reform bill is a pretty good answer to that,” he said. “It is easy to stand on the outside and throw bombs. It is a lot harder to accomplish things in government in a very divided country.”

Trump made the tax plan the primary domestic focus of his entire Cabinet for the past four months. He has drifted far from the populist promises he made on the campaign trail, with his trade agenda stalled and the tax plan tilting more toward benefits for businesses than promised a year ago.

And a lack of focus facing other parts of his job risks lurching the government into immediate dysfunction. Republicans still do not have a plan to fund government operations after Friday, and Democrats have only hardened their opposition to Trump.

“This legislative achievement — as it is — is widely unpopular with rank-and-file voters,” said John Weaver, a veteran Republican strategist and Trump critic. “It, by itself, is not going to give the president any significant boost or change the way people look at him.”

A Quinnipiac University survey last week found that 55 percent of American voters disapprove of the tax plan, with only 26 percent approving. Nearly half of those polled — 43 percent — said they would be less likely to support a lawmaker who voted for the plan.

As the holiday season moves forward, whether the probable win could help President Trump pass other agenda items in 2018 and catapult him onto firmer political ground ahead of next fall’s midterm election is an open question. (Jabin Botsford/The Washington Post)

House Speaker Paul D. Ryan (R-Wis.) has predicted that the bill’s popularity would rise once Americans understand its impact on their lives, as has Trump.

"Stocks and the economy have a long way to go after the Tax Cut Bill is totally understood and appreciated in scope and size," Trump tweeted Tuesday.

The tax plan, assuming it passes, will be the centerpiece of Trump’s uneven first year as president, along with positive economic indicators. The Standard & Poor’s 500-stock index has risen roughly 20 percent since his first day in office, and the unemployment rate has fallen from 4.8 percent to 4.1 percent. Inflation remains low.

Trump’s pick for the Supreme Court, Neil M. Gorsuch, was confirmed by the Senate, as were 12 U.S. Circuit Court judges, the most during a president’s first year in office in more than 100 years.

The administration, meanwhile, has launched an aggressive strategy of dismantling dozens of regulations, and earlier this month Trump took steps to fulfill a campaign promise by vowing to move the U.S. Embassy in Israel to Jerusalem.

It is the tax plan, though, that stands as the most sweeping change, one that could impact nearly every American household and business for much of the next decade. Republicans rallied to support the package, even though a number of them have shown a personal animus toward Trump.

“I hope the lesson is good policy works, better process works,” said Sen. Rob Portman (R-Ohio). “I think the process was far better on this than on health care, and I hope we all learn from that, including the White House. But it’s a victory for him.”

Conservative economist Douglas Holtz-Eakin, a former director of the Congressional Budget Office, said that the White House could follow the same approach if it wanted to pursue changes to entitlement programs such as Medicaid in 2018, as Republicans have spent years working on these issues and the Trump administration has not.

“There is some work on the Hill they could pick up and go with if the administration decided to follow the same model, which is, ‘We will follow your lead, Congress. Go!,’ ” he said.

White House officials and other allies close to Trump believe that the push for tax cuts helped unlock secrets for navigating Congress, including the power of coalescing around a unified message and giving Republican leaders flexibility to cut deals with wavering lawmakers.

They also believe that bulldozing Democrats, none of whom were expected to support the tax plan, gives Trump additional leverage because the tax bill shows he can enact his agenda without help from across the aisle.

“I’ve said for the last two months, if he gets a big legislative victory like this, he’s off to the races,” said Dallas businessman Roy Bailey, who was co-chairman of Trump’s finance committee during the 2016 campaign. “I think he’ll have full wind in his sails.”

But although the administration has a better sense of how Congress works, senior White House officials acknowledged that it remains unclear whether Trump will adjust his controversial style of leadership.

The president showed signs of being easily distracted as the tax plan entered sensitive stages. And he continues to pounce on and verbally destroy critics, such as Sens. Bob Corker (R-Tenn.) and Jeff Flake (R-Ariz.), even when he needs their political help. Trump ended up counting on support from both senators to pass the tax measure.

“The president’s [modus operandi] is to be partisan and to help the wealthy rather than the middle class,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said. “If he changes, we can work with him, but if past is prologue, it’s going to be more of the same.”

The successful, sustained strategy that was on the verge of muscling the tax bill through Congress stands in contrast to incomplete initiatives, such as health care, and unfulfilled campaign promises, such as building a wall at the U.S. border with Mexico. Trump has promised to renew his push for a border wall but has not identified any way to fund the project.

He is not the first president to get off to a rocky start with Congress, though none in recent history has been so unpopular with the general public. Presidential historian Douglas Brinkley likened Trump’s year to 1961, when President John F. Kennedy’s first year in office was beset by failures.

“Kennedy couldn’t get anything big done in Congress,” Brinkley said. “He had the Bay of Pigs. Kennedy did his reset by going to a joint session of Congress and saying we’re going to put a man on the moon.”

For Trump, Brinkley said, passing tax cuts would be “a big moment” similar to Kennedy’s man-on-the-moon promise.

“At the end of the year, he’ll have a major accomplishment,” Brinkley said. “The problem with the tax bill is that it doesn’t quite capture your imagination. It’s not grand history-making. People aren’t going to line up at his presidential museum to learn about a tax cuts bill.”

Erica Werner contributed to this report.