This week's activity in the Senate is likely to determine the outcome of healthcare reform, a top Republican said Monday.

Sen. John Thune (R-S.D.), the chairman of the Republican Policy Committee, said the coming week will seal the fate of Democrats' efforts to pass healthcare legislation.

"We'll see how this week goes," Thune said during an appearance on "Imus in the Morning," broadcast by the Fox Business Network. "I think this week will be pretty determinative of how this thing comes out."

That assertion comes after a rough weekend for Democrats, in which Sen. Joe Lieberman (I-Conn.) notified party leaders that he would oppose the latest iteration of a compromise on the public option provision, which in this case would allow individuals between the ages of 55 and 64 to buy into Medicare.

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"It seems like things are unraveling a little bit for them right now," Thune said.

Lieberman's stance, which has angered liberals within the Democratic Party, won nothing less than praise from Thune.

"God bless Joe Lieberman for being a guy who stands by his convictions," he said.

"What Joe Lieberman has said is that this latest proposal that they put forward is a non-starter with him and he will vote against it, which makes their job of getting to 60 that much more difficult," he added. "They know what it takes to get Joe Lieberman: they've got to drop that idea, but the problem is they lose other people when they start doing that."

Sen. Ben Nelson (D-Neb.) also expressed unwillingness to support the current proposal, particularly if it doesn't include more explicit protections against federal funding for abortions.

That leaves the coming week for Democrats to hash out new details of their health reform plan, especially as they await a score from the Congressional Budget Office (CBO) on the impact of the Medicare buy-in option. A self-imposed deadline of accomplishing health reform by the Christmas holiday late next week also looms on Democrats' horizon.

Thune also briefly addressed a potential dark-horse candidacy for the White House in 2012.

"That job's got a lot of problems," he said of running for president. "I'm happy being the senator from South Dakota. I've got to run for reelection to that job next year, so we're just doing everything we can to slow this trainwreck down in Washington right now."