Senate Republicans on Monday defended their plan to include $1 billion for security funding for President Trump’s ballroom project as they prepared to take up a politically charged budget bill that faces stiff opposition from Democrats.

Returning to the Capitol for the first time since the components of the spending plan were made public, top Republicans said the security money was necessary given the threats to the president, and claimed that none of it would be used for the ballroom itself. The president has said private donations will pay for the ballroom, which he has estimated to cost $400 million, though some Republicans want tax dollars to go to it as well.

“It is a security-related measure,” said Senator John Thune, Republican of South Dakota and the majority leader, who noted the three assassination attempts against Mr. Trump in the past two years. “Obviously the money that’s in there is about securing that building. Secret Service has a job to defend and protect the president, and I need to make sure they have the tools to do it.”

Senator Charles E. Grassley, the Iowa Republican who chairs the Judiciary Committee, said events such as the forthcoming World Cup and the attempted attack at the recent gala for White House journalists demonstrated the need for enhanced security.

“We’ve got to beef up the Secret Service,” said Mr. Grassley, whose panel is responsible for the majority of the funding request.

The funding has come under attack from Democrats who say it underscores the misplaced priorities of Republicans as many Americans struggle with rising fuel prices and higher consumer costs.

“Their billion-dollar bill has zero dollars to help Americans deal with groceries and gas prices going up, zero dollars to bring down Americans’ health care, housing and electricity bills,” Senator Chuck Schumer of New York, the Democratic minority leader, said on the Senate floor on Monday. “At a time when Americans are struggling to put food on the table, Republicans say ‘Let them eat cake,’ and demand American taxpayers build Trump a palace while they are at it.”

Polls have shown that the ballroom project is unpopular, and Democrats have signaled they plan to make it a focus of their election-year message against Republicans. In a letter to Democratic colleagues, Mr. Schumer referred to the G.O.P. majority as “Ballroom Republicans” as he promised to “force vote after vote” to put them on the record about what he called “Trump’s gaudy ballroom.”

The $71.7 billion measure also would provide money for Immigration and Customs Enforcement and Customs and Border Protection for the remainder of Mr. Trump’s term, after Senate Democrats refused to support any spending for those agencies without changes to how immigration officers operate. Republicans are employing a special budget process that can skirt a filibuster to overcome the united Democratic opposition. Republicans want to pass it through the House and Senate before June 1.

Mr. Thune said Republicans have invited the head of the Secret Service, Sean M. Curran, to a private party luncheon in the Capitol on Tuesday to share more information about how the money would be used.

Some Republicans have raised questions about the rationale for the funding.

“I will be following it very closely to see what the justification is for increased security at the White House,” Senator Susan Collins, the Maine Republican in a tough re-election fight, told reporters in Maine late last week. “There, I believe, is a plan for a bunker to be built underneath the ballroom and that may make sense,” she said, adding that she believed the ballroom itself should not be built with federal funds.

Other Republicans, including Senator Lindsey Graham, the South Carolina Republican who chairs the Budget Committee, have called for Congress to approve federal money for the ballroom project.

Republican leaders had pushed to keep the budget measure tightly focused on the immigration enforcement spending in order to make it easier to pass quickly, particularly given the tight vote margin in the House, and the inclusion of the White House money came as something as a surprise.

But Mr. Thune said he did not think it would spur a demand to open the legislation to other proposals.

“I think it fits nicely within a package that is built around basically public safety, whether it’s at the border, on our streets, in our neighborhoods, in our communities — and in this case, it’s protecting the president,” he said.

Democrats criticized the spending plan as outrageous. They noted that Republicans are being forced to try to fund the immigration crackdown outside of regular spending channels because the administration would not come to terms with Democrats on proposals such as banning ICE officers from masking their faces.

“ICE continues to argue it is the only police force in America that should be masked and the American people shouldn’t be able to identify who these ICE agents are,” Senator Richard J. Durbin of Illinois, the No. 2 Senate Democrat, said. “I think that’s reprehensible. We are trying to change it and the Republicans are resisting it.”