President Trump and Senate Republicans will huddle Tuesday on Capitol Hill in hopes of finding something that has eluded them all year: party unity.

Republicans are in broad agreement about the need to pass tax reform, but are struggling to get on the same page as they begin what is likely to be a grueling debate over legislation.

Leading voices in the GOP have said that success on a tax bill could decide whether Republicans sink or swim in next year’s elections, raising the stakes for everyone involved.

Still, Tuesday’s meeting is likely to be awkward.

Two senators who have ratcheted up their rhetoric against Trump in recent weeks, Sens. John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.) and Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.), are likely to be in the room.

Corker recently compared the White House to an “adult day care center,” while McCain over the weekend criticized wealthy people who avoided the Vietnam War because “they had a bone spur.” Trump was granted one of his five deferments from the war because of a bone spur; McCain late Monday denied that the remark was aimed at the president.

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Trump has in the past found it hard to resist getting into a verbal tussle when meeting his critics face-to-face.

The last time Trump sat in on the Senate Republican lunch, in 2016, it was rocky. Trump, who was then the party’s nominee for president, sparred with Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (Ariz.) and criticized Sen. Ben Sasse Benjamin (Ben) Eric SasseChamber of Commerce endorses McSally for reelection Ben Sasse is mistaken with idea for the election of senators in America Big Ten football to return in October MORE (Neb.) and then-Sen. Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (Ill.), whom he reportedly called a “loser.”

Republicans can ill afford to have Tuesday’s lunch turn into an intraparty squabble.

Any three Republican senators together could bring down Trump’s tax plan unless he convinces a few conservative Democrats to support the package. Republicans control 52 seats, so they need 50 votes to pass tax reform without Democratic support, as Vice President Pence would break a tie.

GOP senators want Trump to set aside his feuds and put the full weight of his office behind tax reform. Some lawmakers say he didn’t do a great job at selling the GOP health-care bill, contributing to its failure.

“We’re certainly looking for the president to put the full force and power into helping put through a good tax reform bill,” said Senate Finance Committee Chairman Orrin Hatch Orrin Grant HatchBottom line Bottom line Senate GOP divided over whether they'd fill Supreme Court vacancy MORE (R-Utah).

While Trump was dogged in his pursuit of ObamaCare repeal, he did not seize the bully pulpit by traveling around the country to give speeches on the party’s health-care plan.

Hatch said Trump should sell tax reform every chance he gets.

“It’s always an issue. No matter what you do, you never say enough about it,” Hatch observed about some of his colleagues’ grumbling during the health-care debate.

Trump has shown he has learned his lessons from the ObamaCare debate. He has already put in more legwork selling his tax plan to the public and fellow Republicans on Capitol Hill.

The president joined a conference call on Sunday afternoon with House Republicans, urging them to immediately pass the Senate-approved budget resolution and get to work on tax reform.

He invited Republican and Democratic members of the Senate Finance Committee, the Senate’s chief tax writing panel, to meet with him at the White House last week.

Trump touted his tax plan in a Sunday op-ed published in USA Today, promising “it will raise the annual income of a typical hardworking American household by an average of around $4,000.”

And he sat down with Maria Bartiromo of Fox Business Network at the White House Friday to pledge that Congress will pass the “biggest [tax] cuts ever in the history of the country” and predicted it would happen “hopefully before the end of the year.”

Senate Republicans expect to hear a similar pitch from Trump on Tuesday when they gather in the historic Mansfield Room.

Trump may also touch on the backlog of administration nominees, something he and Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE (R-Ky.) discussed last week at the White House.

Senators may bring up other concerns, such as immigration, spending bills or the Iran deal, say GOP aides.

But GOP lawmakers are most eager to hear from Trump on taxes.

“The main thing is to keep the main thing the main thing: tax reform,” said Sen. Steve Daines Steven (Steve) David DainesMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Toobin: McConnell engaging in 'greatest act of hypocrisy in American political history' with Ginsburg replacement vote MORE (R-Mont.). “We should entitle this bill the American Jobs Act.

“The end is growing the economy and creating more jobs,” Daines said.

Republican senators want to pass a robust tax package by the end of the year or early next year so that it will have a chance to boost the economy ahead of the 2018 midterm elections.

But as Democrats try to paint tax reform as a giveaway to the wealthy and corporations, GOP leaders say it will be crucial for Trump to emphasize the benefits of the plan to the middle class.

“He just has to continue to drive home his desire in signing a bill that really does deliver relief to middle-income families,” said Senate Republican Conference Chairman John Thune John Randolph ThuneGraham: GOP will confirm Trump's Supreme Court nominee before the election Tumultuous court battle upends fight for Senate What Senate Republicans have said about election-year Supreme Court vacancies MORE (S.D.), the third-ranking member of the leadership.

To do that, “it’s going to take some significant offsets,” Thune warned.

“He’s got to be very forceful that we’ve got to get this across the finish line and we have to make some hard decisions,” he cautioned, referring to various popular tax breaks that would be ended to raise revenue to lower individual and corporate tax rates.

The president told Fox Business Network Friday that he would prefer that the congressional tax writing committees not create a fourth income bracket to limit the benefits of the tax package for people earning more than $1 million a year. On Monday morning, he also ruled out changes to 401(k) plans, something Republicans had reportedly been eyeing to help pay for the tax overhaul.

Trump is not scheduled to speak to reporters alongside McConnell at the weekly media stakeout that usually follows the Senate Republican lunch.

The president made a show of burying the hatchet with McConnell last week, inviting him to lunch at the White House. He then appeared with the leader at a joint press conference in the Rose Garden at which they pledged to back the same agenda.

The event was aimed at easing what has been a rocky relationship between Trump and Senate Republicans this year.

In August, Trump repeatedly blasted McConnell for failing to deliver legislation to repeal and replace ObamaCare. At one point, he suggested McConnell should step aside if he couldn’t deliver.

Trump’s criticism of the Senate leader threatens to become a full-fledged revolt in next year’s elections, when Trump’s former adviser, Stephen Bannon, hopes to support candidates who can defeat members of the GOP establishment.

The president has already signaled his desire to oust Flake, a longtime critic he once called “toxic.” Trump encouraged a conservative former state legislator, Kelli Ward, to challenge Flake in next year’s primary.