The prospects of Congress passing comprehensive immigration reforms that provide instant legal status and a pathway to citizenship for illegal immigrants will depend largely on the Senate, where Republicans and Democrats have been slow to offer their support for such changes.

"I'll have to wait and see," said Sen. Max Baucus, D-Mont., who joined 14 other Democrats in 2007 to help kill the Senate's last effort to pass comprehensive immigration reform. That 2007 proposal, from then-President George W. Bush, was similar to the latest plan offered Monday by a group of bipartisan senators.

As he did six years ago, Baucus must decide whether to back a bill that grants legal status to 11 million immigrants now living illegally in the United States, a change that Baucus and others labeled amnesty for lawbreakers. And just like six years ago, he's going to have to decide just before he faces re-election.

"People in Montana don't want amnesty," Rosemary Jenks, a lobbyist for NumbersUSA, which seeks to lower the nation's immigration level, told The Washington Examiner.

Jenks believes Baucus and other Democratic senators up for re-election next year, including Mark Pryor of Arkansas and Mary Landrieu of Louisiana, will join with a majority of Senate Republicans to help kill the plan.

"I don't even think it's going to get through the Senate," Jenks said of the legislation.

President Obama on Tuesday unveiled his own immigration reform plan, one that would grant citizenship more quickly and provide less for border security than the Senate plan. And Sen. Marco Rubio, R-Fla., one of the eight senators who crafted the bipartisan compromise, said he was "concerned" those differences could doom the chances of passing immigration reform this year.

House leaders, meanwhile, may let the Senate act first before taking on immigration reform in their own chamber.

"My sense is it's going to move in the Senate before it moves in the House," said House Energy and Commerce Committee Chairman Fred Upton, R-Mich.

Upton said lawmakers in both parties are eager to pass some kind of immigration reform legislation because "it's a broken system." But it will be difficult to build Republican support for a bill they see as providing amnesty.

"It depends on how it's all defined," Upton said.

Obama in his immigration speech Tuesday pledged to send his own bill to Capitol Hill if the House and Senate don't move quickly to pass their own legislation. But it could take lawmakers a while just to craft the legislation given that disagreements exist between the senators who helped craft the framework for the bill.

Sen. Charles Schumer, D-N.Y., declared Monday that illegal immigrants would gain legal status the moment a bill becomes law. A day later, Sen. John McCain, R-Ariz., claimed they wouldn't.

"I'm not ready to move forward until we have the border operationally secure," McCain said.

When those kinds of disagreements exist, Sen. Bob Corker, R-Tenn., said, even a much ballyhooed plan could quickly die on the Senate floor.

"The last time we tried this [in 2007], between the talking points and the legislation, things kind of broke down," Corker said. "The details really matter on something like this."