On a day when the Senate Republican Conference is expected to approve a two-year ban on earmarks for its members, possibly even by voice vote, Fox has learned that a bipartisan Senate duo is expected to announce a challenge to the full chamber to do the same.

As early as Wednesday, Sens Claire McCaskill, D-Mo., and Tom Coburn, R-Okla., are expected to offer an amendment to a food safety bill that would put members on record on the moratorium.

McCaskill released a statement Monday applauding the dramatic reversal of Senate GOP Leader Mitch McConnell, R-Ky, in support of a ban on pet projects. In that statement, McCaskill, a close ally of the White House who has never requested an earmark since she was elected to Congress in 2006, said, "I'm glad that Republican leadership is coming around to this idea; now it's my Democratic colleagues turn to get on board."

Coburn and McCaskill are not expected to get a vote on their amendment, which was filed late, but a senior Senate GOP aide said the two could request a roll call vote on their motion to take up the amendment. That, the aide said, would serve as an "on the record vote" on the ban.

McCaskill said she and Coburn will now be joined by Sens. John McCain, R-Ariz, and Mark Udall, D-Colo.

The Missouri senator told reporters Tuesday, "This should be the easy stuff. I mean, we still have the hard stuff like debt and deficits."

McCaskill said she has "asked Santa for this on her Christmas wishlist this year. And I feel better that I might get it this time."

But not many of McCaskill's Democratic colleagues are jumping at the chance to ditch their pet projects.

The Senate's number two Democrat, Dick Durbin of Illinois, said merely, "I'll take a look at it."

Sen. Ben Nelson, D-Neb., said flat out, "I don't think much of that at all," calling it a "cruel hoax" to say it cuts deficits when the money is not removed from the budget and used to cut the deficit.  "It's definitely shady," Nelson added.