The Senate vote this week on the budget blueprint of Rep. Paul Ryan (R-Wis.) will be a gut-check moment for Republicans, who risk angering constituents no matter what they do.

After seeing GOP presidential hopeful Newt Gingrich get pummeled for calling the House Budget chairman’s plan to overhaul Medicare “right-wing social engineering,” Republicans have begun to realize that opposing Ryan does not come without a cost from the party’s base.

But changing Medicare into a subsidy for private insurance is a big loser in general election polls, and Ryan’s plan is giving Democrats new hope that they will take back the House and retain the Senate in 2012.

The upcoming Senate vote puts vulnerable GOP incumbents, such as Sens. Olympia Snowe (Maine) and Dick Lugar (Ind.), both of whom face primary challenges, in a particularly tight spot. Several others remain undecided, including Sen. Lisa Murkowski (Alaska), who has broken with her party on several key votes.

Lugar said Monday that he would vote for Ryan’s budget.

“I support the creative thinking … expressed by the Congressman,” Lugar said. “Creativity is going to be required to save Medicare.” He noted that under Ryan’s plan, people 55 and older would stay in traditional Medicare.

Sen. Scott Brown, however, appeared to do an about-face on the issue, announcing in an opinion piece in Politico that he would vote “no” after he had been quoted in the Newburyport Daily News saying he would support it.

Medicare changes are needed to cut costs, “but I do not think it requires us to change Medicare as we know it,” the Massachusetts Republican said. The language that he used, some Democrats pointed out, appeared to come verbatim from their talking points.

But Sen. Rand Paul, who was elected under the tea party mantle last year, said that he expects not only to get a vote this week on his plan to balance the budget in five years but also that he would vote against Ryan’s plan. The Kentucky Republican said Ryan’s budget doesn’t reach balance until the late 2030s and adds trillions in debt over the next decade.

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

Democrats aren’t shy about saying the vote on the Ryan budget this week is intended to set up Republicans politically for 2012. Senate Majority Leader Harry Reid (D-Nev.) plans to bring up the House-passed budget for a vote even though he is not obligated to do so and has said it would be “foolish” for Democrats to put forward a budget of their own.

“This issue will have staying power and be a defining issue in 2012,” Democratic Conference Vice Chairman Charles Schumer (N.Y.) said in a conference call Monday with reporters.

The issue will show voters “why we need to keep the Senate Democrats in order to counter House Republicans. … The Republicans tried to end Medicare, but a Democratic majority stopped it in the Senate. … It’s that simple,” Schumer said.

The issue is already paying dividends for Democrats, he said, noting that the Medicare issue has helped boost the Democrat running in the special election in New York, despite the GOP tilt of the district.

“Senate Republicans clearly could not be more nervous,” Schumer said. “Sen. Scott Brown backtracked. We think other Republicans are likely to follow.”

Republicans want to force a vote on another alternative budget by Sen. Pat Toomey (R-Pa.) that cuts spending even deeper than the Ryan plan would, Schumer said. Toomey’s plan does not include Ryan’s Medicare proposal, but Schumer predicted that the gambit wouldn’t work.

The public would know “which side put Medicare on the chopping block, which side stood up to defend it,” he said.

Republicans pointed to the Democrats’ decision not to offer a budget of their own. Minority Leader Mitch McConnell (R-Ky.) has said he plans to force the chamber to vote on President Barack Obama’s budget in an attempt to embarrass Democrats.

Republicans also ripped Democrats for playing pure politics.

Schumer “said himself that his decision to push for the vote has everything to do with the [Democratic Senatorial Campaign Committee] and nothing to do with fixing our debt, which is the sort of approach that got us into this mess in the first place,” a Senate GOP aide said. “Americans want politicians to stop playing grab-ass with pollsters and start getting serious about the budget.”

Senate Budget ranking member Jeff Sessions said Monday that he will object to any unanimous consent request to adjourn for the Memorial Day recess unless Democrats bring forward their own budget plan.

“I’m going to require that we have a vote on it,” the Alabama Republican said. “Let’s vote to go home having not done the people’s business,” he added sarcastically.

Republicans argued that without an alternative of their own, Democrats were voting to let Medicare go bankrupt.

“The current plan on the other side from the president and Harry Reid and others is we are going to ration care and essentially this thing will go into bankruptcy. That’s all we have heard, rationing and bankruptcy from the president and Harry Reid,” House Majority Leader Eric Cantor (R-Va.) told reporters Monday.

Schumer dismissed the Democrats’ lack of a budget blueprint — saying that could come later after bipartisan talks with Vice President Joseph Biden conclude.

“The only way we are going to get a real budget is by Democrats and Republicans sitting down together,” Schumer said.

The Biden negotiations are set to resume today, and Congressional aides said the group will finally begin to discuss Medicare and Medicaid.

“They are going to move on to trickier topics,” an aide familiar with the negotiations said. “It doesn’t mean they won’t make progress. It just gets slower now.”

Anna Palmer and David M. Drucker contributed to this report.