A Democratic senator facing a tough reelection battle in 2012 said on Wednesday that she would oppose a $1.1 trillion spending bill before Congress.

Sen. Claire McCaskill (D-Mo.) said she'd break with her party to oppose the omnibus legislation crafted by Democratic leaders, citing concerns over earmarks contained in the legislation.

"It'll be interesting to see — I'm voting no," she said in a conference call with Missouri reporters. "The omnibus is a name for the giant spending bills that come through at the end of the year that are usually laden with earmarks. So it'll be interesting to see if they can get the votes for this."

McCaskill has voted against previous omnibus deals, but her obstinate opposition threatens defeat for the spending package unveiled yesterday by Senate Democrats. Republican senators have loudly criticized the omnibus deal, and if all 42 GOP senators are joined by some Democrats, there may not be enough votes to advance the package.

The alternative option would be a continuing resolution, which pegs spending at a lower level, and would punt the issue of government spending to an empowered GOP in the next Congress.

The stubborn opposition to earmarks also allows McCaskill a degree of political opportunity back home in Missouri, a state that elected her in 2006 and has tended toward Republican candidates since.

She took jabs on Wednesday at Senate Minority Leader Mitch McConnell (R-Ky.), who has earmarks in the bill despite his vow to defeat it, as well as Sen.-elect Roy Blunt (R-Mo.), who's been an ardent proponent of the practice.

McConnell urged lawmakers to pursue a continuing resolution instead of an omnibus on Wednesday morning.

“I think a more appropriate approach is to pass a sensible, short-term CR that gets us into next year, when the new Congress will have the opportunity to make a determination on how to best spend taxpayers’ money. The government runs out of money this Saturday. Congress should pass a short-term CR immediately," he said on the Senate floor.

Updated 12:07 p.m.