President Trump on Wednesday took his case for massive tax cuts directly to the public, even as Senate Republicans struggle to unite behind the proposal ahead of a key Senate vote that could derail his entire approach.

During a speech to truckers and others here, Trump touted what he alleges are the tax cuts’ benefits for the working and middle classes, making grand-but-disputed claims about the additional dollars workers would see if tax rates were cut for corporations.

“You’re going to make more money, you’re going to do better than ever before, and we truly admire you,” Trump told the truckers assembled before him. “You are our heroes.”

Trump’s speech came just one week before Senate Republicans must decide whether to pave the way for his tax plan. They are planning to vote on a budget that would allow the tax cuts to raise the deficit by as much as $1.5 trillion over 10 years. At least one Republican, Sen. Rand Paul (R-Ky.), is expected to oppose the budget resolution, as he has traditionally voted against any budget that doesn’t eliminate the deficit.

Republicans control only 52 of the Senate’s 100 seats, and they need at least 50 votes to advance the budget, as Vice President Pence would vote to break any tie. And if the House and Senate don’t pass matching budget resolutions, then the Senate cannot approve tax cuts with a simple majority.

That would require them to seek help from Democrats, who so far have mostly opposed the construct of the GOP tax plan on the grounds that it would largely benefit the wealthy while potentially raising taxes for some others.

With Paul’s support considered unlikely, Republicans can afford to lose at most one more vote if they hope to move the budget forward. Sen. John McCain (R-Ariz.) opposed the White House’s effort to repeal the Affordable Care Act, and his support for the budget is unclear. Sen. Susan Collins (R-Maine) has also bucked the White House in the past and is going to announce on Friday whether she will run for reelection to the Senate or for governor. That decision could influence how she approaches legislation going forward.

Meanwhile, Sen. Thad Cochran (R-Miss.) has been ill and hasn’t cast a vote since mid-September. Republicans hope he will be back by next week, but that is uncertain. And Trump’s public and messy name-calling exchanges with Sen. Bob Corker (R-Tenn.) in recent days have also raised concerns within the GOP about whether his support for the budget will waver.

The budget resolution would essentially allow the tax-cut plan to add $1.5 trillion to the debt over 10 years, and Corker has said he wants assurances that it would actually reduce the deficit, not increase it. If Corker decides to buck GOP leaders on the Senate budget resolution vote, it could imperil the entire tax plan.

“It’s not something they are taking for granted,” said Douglas Holtz-Eakin, a conservative economist who is president of the American Action Forum, referring to Senate Republican leaders’ assessment of the upcoming budget vote. “They are worried.”

Until recently, Senate Republicans were touting progress on the budget deal as a sign of their party’s ability to unify, after party deficit hawks found a compromise with those pushing for the large, supply-side tax cuts to move the bill through committee. But the budget’s prospects in the broader Senate again have become clouded.

The White House and Republican lawmakers are under a tremendous amount of pressure to pass the tax-cut plan, having come up short on other priorities earlier in the year. The House already has passed a budget resolution that is very different from the one Senate lawmakers will consider next week, but leaders have expressed confidence that they can work out their differences.

For Congress, Trump also offered a blunt message about his plan: “All I can say is, you better get it passed.”

“They will. I know,” he added.

Trump spoke Wednesday in front of an open airport hangar door, with a semi-truck visible stage right and another behind him on the tarmac. The truck behind him was emblazoned with a custom logo, “Truckers for Tax Reform,” inside a large red circle next to the words “win again” and “lower taxes, bigger paychecks, more jobs.”

Corralling Senate Republicans for the budget vote is just one part of the process that the White House is trying to pin down. Trump is also trying to convince supporters of the plan’s benefits even though many details remain unresolved. He has said that a massive tax-cut package will speed up economic growth and lead to more investment in the United States, raising wages, with workers seeing an ­additional $4,000 in pay over eight years.

Trump based this assertion on rough findings from his Council of Economic Advisers chairman, Kevin Hassett, who has a contrarian view from other economists and says that corporate tax cuts primarily help workers, not companies.

Democrats are skeptical of Hassett’s claim, arguing that the plan, as currently designed, would add to the debt and primarily help the wealthy. Some party members on Wednesday asked how Trump could claim the tax plan would raise wages by $4,000 over eight years when specifics of how the plan would work haven’t been released.

“I heard that number, and I have not seen any evidence that even comes remotely close to that suggestion,” said Rep. Richard E. Neal (D-Mass.), the top Democrat on the House Ways and Means Committee.

In a nod to the sway some Democrats might ultimately have on the tax plan’s fate, Trump has been pitching it in states that he won last year and where vulnerable Democratic senators are up for reelection next year.

Trump was joined at his events in North Dakota and Indiana by Sens. Heidi Heitkamp (D-N.D.) and Joe Donnelly (D-Ind.) and made clear that he wants their support for his tax cuts. At the event in Indianapolis, the president motioned to Donnelly and told a supportive crowd that “we’ll campaign against him like you wouldn’t believe” if he isn’t on board.

Sen. Robert P. Casey Jr. (Pa.), a member of the Finance Committee, is one of 10 Democrats in the chamber who are up for reelection next year in states that Trump won. Casey did not attend Wednesday’s event in Harrisburg.

His tax message is also being reinforced on the ground by operations set up by the Republican National Committee in 19 states, including Pennsylvania. The RNC, buoyed by robust fundraising this year from small donors, has targeted state party leaders it believes will be key to the GOP’s fate in next year’s midterms as well as Trump’s reelection bid.

Beyond preparing for elections, the field operations set up by the party are working to promote Trump’s tax initiatives with campaign-style tactics, including door knocking, phone banks and house meetings, RNC staff say.

Mike DeBonis, Paul Kane and Kelsey Snell contributed to this report.