WASHINGTON (Reuters) – A public feud between U.S. President Donald Trump and Senate Republican maverick Bob Corker could narrow the path for tax reform in the U.S. Senate, where a Republican go-it-alone effort is already showing signs of disunity.

Days after the Republican-controlled Congress took two important steps toward advancing tax legislation, a Trump-Corker shouting match threatened to further alienate Trump from key Republican senators such as John McCain, who prevented the party earlier this year from repealing Obamacare.

The political stakes could not be higher for Republicans. Tax reform offers them a chance to show they can govern and woo voters for the November 2018 midterm elections. Failure could mean losing control of the House of Representatives.

Trump and top Republicans have unveiled a plan to slash taxes for businesses and individuals, the first comprehensive overhaul of the U.S. tax code since 1986. They hope to complete the monumental task by January.

Tax legislation could move quickly once the House and Senate agree on a fiscal year 2018 budget resolution, according to analysts who expect the Senate to vote on a measure next week and say the House could approve it soon thereafter.

The Republican plan calls for the House to vote on tax reform this month, House Speaker Paul Ryan said in a recent television interview.

But the tax reform push has been dogged by delays and distractions such as Trump’s criticisms of party leaders including Senate Republican leader Mitch McConnell and Ryan.

“Trump is running an outside game working to appeal to his core base of support and doesn’t necessarily care how this may or may not affect his relationship with the Senate,” said Republican strategist Ron Bonjean.

The latest incident erupted on Sunday. Trump lashed out at Corker on Twitter, saying the lawmaker had begged the president for his endorsement and announced his retirement after being turned down.

Corker replied in his own tweet describing the White House as an adult day care center and told the New York Times that Trump risks setting the nation on the path to “World War Three.”

The spat is exactly what Republicans do not need as they move tax legislation through the Senate, which they control by only a 52-48 margin. Most Democrats are united in opposition to the plan, and Republicans cannot pass it if they lose support from more than two lawmakers of their own party.

“This is a delicate balance,” said Stephen Moore, a fellow for the conservative Heritage Foundation who helped write Trump’s campaign tax plan. “It all comes down to whether you can get 50 votes in the Senate. Right now, by my count, they’re at about 48. A few votes short.”

Divisions have emerged over proposals to repeal the federal inheritance tax and a state and local taxes deduction. Senate Republican Rand Paul has expressed unhappiness over reports that Trump’s tax plan could raise taxes on some middle-class Americans.

A key player in the tax debate, Corker helped the Senate move closer to legislation by agreeing to a budget resolution that would allow tax reform to lose $1.5 trillion in revenue.

But he has vowed not to vote for any tax package that adds to the federal deficit. Analysts say Trump’s derisive tweets do not encourage cooperation, especially now that the Tennessee Republican has announced his retirement.

“Bob Corker, at this point, is as free as John McCain is to do what he thinks is right,” said William Galston, former domestic policy adviser to President Bill Clinton.

Republicans dismissed Trump’s fight with Corker.

“Most aren’t retiring and know they must still work with the White House or answer to frustrated voters,” Bonjean said.