(By Joshua Roberts/Bloomberg)

Maine Republican Sen. Susan Collins, who had announced last month that she opposed the House Republican budget proposal, confirmed Monday that she would vote against the plan this week, telling reporters, “I can’t support it in its present form.”

Kentucky Republican Sen. Rand Paul also said Monday that he would vote against the budget proposal. But unlike some of the more moderate senators who are opposing the plan, Paul, a conservative freshman and founding member of the Senate Tea Party Caucus, told reporters that he would not back the plan because it does not cut enough from the federal budget.

“I can’t vote to add that much debt,” Paul said.

The Senate is likely to vote Thursday on two competing budget proposals: the Ryan budget and the plan put forth by President Obama in a George Washington University speech. Leaders in both parties are likely to see some of their caucus members vote against their party’s plan, although moderate Republicans in particular are in a politically vexing spot due to Democrats’ relentless campaign-trail focus on Medicare, which the Ryan budget plan would significantly overhaul.

Other moderate senators who had not stated how they planned to vote as of Monday evening included Republican Sens. Olympia Snowe (Maine), Mark Kirk (Ill.) and Lisa Murkowski (Alaska).

Even as some Republican senators stated Monday that they would vote against the Ryan plan, other potentially vulnerable members defended the proposal. Sen. Dick Lugar (R-Ind.), who faces a competitive primary in his bid for reelection in 2012, told reporters that he would back the plan, as did Sen. Kelly Ayotte (R), a freshman from New Hampshire.

“If the Ryan budget comes up for a vote in the Senate, I will support it,” Ayotte said. “And I will say this: I would like to see if those who have been so critical of this budget to put forward a constructive plan. What’s your plan for America? ... Now is the time for leadership in this country, and I respect that Paul Ryan has shown that leadership, and there’s no other plan on the table right now that puts us on a sustainable path, addressing the rising costs of Medicare.”

Lugar said Monday that he deemed the House Republican plan a “constructive” budget and that he believed Ryan has “had the courage to indicate that everybody over 55 will continue as they are; those who are younger will have to be involved in some new thinking.”

“There are no other alternatives thus far, but in any event, I support the creative thinking that has been expressed by the congressman,” he added.

Staff writer Paul Kane contributed to this report.