GOP Sen. Scott Brown (Mass.) said Monday he won't support Rep. Paul RyanPaul RyanThe Hill Interview: Budget Chair Black sticks around for now Gun proposal picks up GOP support GOP lawmaker Tim Murphy to retire at end of term MORE's (R-Wis.) budget when it comes up for a vote in the Senate.

Brown, a centrist who is running for reelection in 2012, said that Ryan's plan helped jumpstart a necessary debate, but that its Medicare reforms go too far.

"While I applaud Ryan for getting the conversation started, I cannot support his specific plan — and therefore will vote 'no' on his budget," he wrote in a Politico op-ed.

"Our country is on an unsustainable fiscal path," he added. "But I do not think it requires us to change Medicare as we know it. We can work inside of Medicare to make it more solvent."

Brown follows centrist GOP Sen. Susan CollinsSusan Margaret CollinsGun proposal picks up GOP support Giffords, Scalise highlight party differences on guns Agricultural trade demands investment in MAP and FMD MORE (Maine), who announced last month she will not vote for the plan

Senate Majority Leader Harry ReidHarry ReidChris Murphy’s profile rises with gun tragedies Republicans are headed for a disappointing end to their year in power Obama's HHS secretary could testify in Menendez trial MORE (D-Nev.) is expected to hold a vote soon, possibly this week, on the plan. The plan is expected to fail, but Reid has said it could elucidate divisions within the Republican Party.

Democrats have seized on Ryan's Medicare plan, criticizing it as an extreme proposal that would privatize the popular, but fiscally troubled, entitlement program. Recently, they have dubbed it the GOP's "litmus test," playing up incidents where Republicans have split with the plan.

All but four House Republicans voted for Ryan's budget.

Ryan's Medicare plan would transform the entitlement program into a "premium support" system that would provide aid to seniors to buy certain private health insurance.

There had been some confusion over Brown's position: He recenty indicated to a Massachusetts business group he would vote for the plan.

"The leaders will bring forward [Ryan's budget] budget, and I will vote for it, and it will fail," he said. "Then the president will bring forward his budget, and it will fail."

Brown's team later said he misspoke and was simply commenting on the political atmosphere.

The Democratic Senatorial Campaign Committee (DSCC) pounced on Brown's statement, claiming his stance on Medicare reform still is not clear.

“Scott Brown has lost his barn coat sheen. He talks like a D.C. politician who is trying to have it both ways and hide his own extreme positions,” said DSCC spokesman Matt Canter. “Brown’s op-ed today was certainly a tantalizing read, but there is still so much that Brown is refusing to tell his constituents about where he stands when it comes to Medicare and protecting seniors.”

Brown said Monday that Medicare needs to be reformed, but that Ryan's plan places a disporportionate burden on seniors. He said he favors eliminating waste, fraud and absuse and providing tighter oversight of providers.

"Seniors should not have to bear a disproportionate burden," he wrote. "But that doesn’t mean we do nothing. If Medicare is to survive for current beneficiaries and future generations, we must act. The sooner Congress addresses this, the less painful it is likely to be — but more difficult adjustments will be required if we delay."