House Speaker Paul Ryan of Wis., pauses as he speaks with reporters on Capitol Hill in Washington, Tuesday, March 21, 2017, after meeting with President Donald Trump who came to Capitol Hill to rally support among GOP lawmakers for the Republican health care overhaul. (AP Photo/J. Scott Applewhite)

House Speaker Paul Ryan of Wis., pauses as he speaks with reporters on Capitol Hill in Washington, Tuesday, March 21, 2017, after meeting with President Donald Trump who came to Capitol Hill to rally support among GOP lawmakers for the Republican health care overhaul. (AP Photo/J. Scott Applewhite)

WASHINGTON (AP) — The vote on the Republican health care bill is a defining moment for House Speaker Paul Ryan that could boost his aggressive agenda to overhaul the tax code and remake the federal government.

Or send it off the rails.

If he fails? “It will be very hard to manage this,” the Wisconsin Republican told reporters ahead of Thursday’s likely vote.

The bill would repeal major parts of former President Barack Obama’s health law, capping future funding for Medicaid and cutting tax increases for high-income families and health insurance companies. It is the kind of high-impact legislation that has become rare in a Congress that sometimes struggles with routine duties like keeping the government open.

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Highlighting the urgency of passing the bill, President Donald Trump spent some of his own political capital Tuesday when he visited Capitol Hill to urge House Republicans to vote for it. Trump warned GOP lawmakers that they could lose their majorities in the House and Senate in the 2018 midterm elections if they renege on promises to repeal and replace Obama’s signature health law.

“If it fails, then there will be a lot of people looking for work in 2018,” said Rep. Mike Conaway, R-Texas.

“This is a domino effect. If we don’t get this done, then you don’t move to tax reform with the same kind of oomph,” Conaway added. “This is the first big test that we’ve got to get done and that’s pretty clear to most everybody.”

Success has consequences, too, because Republicans would then own America’s health care system. Ryan, his party’s vice presidential nominee in 2012 and perhaps a future White House hopeful, understands the stakes, calling the vote “a rendezvous with destiny.”

Some conservative House members think the health bill is too much government regulation and too generous. Many of these same lawmakers essentially forced out Ryan’s predecessor as speaker, John Boehner of Ohio.

In interviews Tuesday, no lawmakers suggested that Ryan could lose his post over the vote. But many worried that the Republican agenda is at stake.

The political marriage of Ryan and Trump is an odd pairing as the speaker only reluctantly backed Trump last year and distanced himself from the presidential nominee after the audio emerged in October of Trump boasting about groping women. But they joined forces after the election.

Ryan and Trump now have an aggressive agenda for Trump’s first year in office. Ryan wants to re-write the tax code for the first time in 30 years after passing the health bill.

Under Ryan, the House has passed significant legislation, including a financial rescue package for Puerto Rico, ending a ban on crude oil exports and passing a law to deal with opioid addiction.

But Ryan’s dream of passing monumental legislation to change America’s big benefit programs has so far been elusive. Over the years, Ryan has laid out proposals to revamp Medicare and Medicaid. The proposals came with catchy titles like “A Better Way” and “The Path to Prosperity.” But until now, they were more campaign slogan than legislation with a chance to become law.

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In the past, Republicans could blame Senate Democrats or Obama for blocking their ideas.

Not anymore.

Republicans control the House, Senate and the White House.

Still, Ryan has to navigate a House Republican conference that is not of one mind on health care. And if he’s successful, the bill would go to a Senate that is still short on votes to pass it.

Some moderate Republicans worry that too many people would lose health coverage under the bill. Over time, 24 million fewer people would have health insurance under the GOP plan, according to the non-partisan Congressional Budget Office.

“Speaker Ryan has certainly put his speakership on the line with the president, saying he will deliver the vote on Thursday,” said Rep. Chris Collins, R-N.Y. “We all campaigned on repealing and replacing Obamacare, each and every one of us. And yet we have a group of members threatening to continue Obamacare.”

Some conservatives don’t want to pass anything that looks like a new entitlement program. Some have dubbed the bill “Obamacare Lite,” and they warn that passing it will doom the Republican Party.

“It’s really a make-or-break moment for conservatives and Republicans,” said Rep. Thomas Massie, R-Ky. “If this passes, it does not bode well for conservatives in the House.”

Another conservative, Rep. Jim Jordan, R-Ohio, said, “The president did a great job” of selling the bill. But, he added, “The bill’s still bad.”

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Associated Press writers Kevin Freking, Richard Lardner and Matthew Daly contributed to this report.

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Follow Stephen Ohlemacher on Twitter at: http://twitter.com/stephenatap