President Bush's plan to overhaul Social Security fared poorly on Tuesday in a test vote on Capitol Hill, with the Senate splitting 50 to 50 on a nonbinding measure declaring that Congress should reject any Social Security plan that would require "deep benefit cuts or a massive increase in debt."

Five Republicans joined the Senate's 44 Democrats and one independent in voting for the resolution, a symbolic effort to demonstrate opposition to Mr. Bush's plan to allow workers to invest part of their taxes in private retirement accounts. Although the measure failed with one vote short of a majority, Senator Charles E. Schumer, the New York Democrat who has been a leading opponent of the plan, later said it was a "significant vote." 

"I think this is another marker on the road to rejection of the president's plan," Mr. Schumer said. At the same time, the Senate made it clear that Mr. Bush, who has made overhauling Social Security his top domestic priority, has succeeded in turning the retirement program into a pressing national issue.

Lawmakers voted unanimously, 100 to 0, to approve another nonbinding measure, one declaring that strengthening Social Security is "a vital national priority."

The votes were part of a string of amendments to a $2.57 trillion budget resolution for 2006 that the Senate is considering. Lawmakers hope to adopt the budget by the end of the week, but doing so will be difficult because the measure is stuffed with contentious issues like reducing spending on entitlement programs like Medicaid, extending Mr. Bush's tax cuts for the next five years and drilling for oil in the Arctic National Wildlife Refuge.

The House and Senate budgets differ sharply on how to control spending, so much so that the chairman of the House Budget Committee warned Tuesday that adopting a budget this year might be impossible. 

The chairman, Representative Jim Nussle of Iowa, said he did not see how the two chambers could come to terms on spending reductions in Medicaid and other entitlements. In the House, which is to take up its budget resolution on Wednesday, Republicans are proposing $68 billion in entitlement cuts for 2006, $18 billion more than Mr. Bush has requested. 

The Senate is proposing $32 billion in cuts, including $14 billion in spending reductions on Medicaid - and some Senate Republicans are balking at that. For instance, Senator Gordon H. Smith, Republican of Oregon, is proposing to strike the $14 billion in Medicaid cuts and instead create a bipartisan commission to study the issue. 

"I think the real challenge for us is the Senate," Mr. Nussle said. 
He added, "We are not going to budge when it comes to controlling spending." In the Senate, Mr. Smith's proposal for a Medicaid commission appears to be generating support among some of his Republican colleagues, who are feeling pressure from home-state governors. 

"This is a very logical approach," said Senator Mike DeWine, Republican of Ohio, who said the Medicaid cuts would be devastating for his state. 
The vote on oil drilling in the Arctic, expected as early as Wednesday, will be especially close, both sides say. Senate Republicans predicted on Tuesday that they had enough votes to push through budget language that would clear the way for opening up the refuge to drilling, a central component of Mr. Bush's energy policy.

"My sense is the votes are there," said Senator Judd Gregg, the New Hampshire Republican who is chairman of the Budget Committee.

By including a drilling provision in the budget, Senate Republicans can avoid the threat of a Democratic filibuster, which has blocked the plan in the past. 
At a packed news conference on Tuesday, opponents of drilling denounced the tactic as "an end run" around Senate procedure, in the words of Senator Maria Cantwell, Democrat of Washington. Ms. Cantwell is trying to strike the drilling language from the budget.

The debate revolves around 1.5 million acres of the 19-million-acre wildlife refuge. Proponents contend that drilling would reduce American dependence on foreign oil; opponents say it would destroy one of the last unspoiled wildlife habitats in the United States. On Tuesday, lobbyists for environmental organizations flooded the Senate, trying to buttonhole lawmakers to vote against the measure.

Among those they are focusing on is Senator Mel Martinez, Republican of Florida, who has been a staunch opponent of drilling in his home state. But Mr. Martinez said he would vote in favor of drilling in Alaska, even though the environmentalists have been running advertisements against him at home.

"I am feeling the heat," he said.

A vote is also expected Wednesday on including financing for Amtrak in the budget against the wishes of the Bush administration, which wants to reorganize the railroad through bankruptcy if Congress does not restructure it. 

Amtrak supporters staged a rally on Tuesday, saying talk of bankruptcy might become a self-fulfilling prophecy, even if Congress eventually allocated money. 

"You talk bankruptcy, and you cause bankruptcy," said Patrick H. Hays, the mayor of North Little Rock, Ark., speaking on behalf of the United States Conference of Mayors.

Matthew L. Wald contributed reporting for this article.