The Democratic tax plan, called the Middle Class Tax Cut, didn’t address the estate tax, but leadership suggested that the Senate would take up the issue separately. If Congress doesn’t act on the estate tax by the end of the year, the current rates of 35 percent on estates worth more than $5 million would expire and revert back to a level of 55 percent taxation on estates of more than $1 million.

The group's estate tax bill, S. 3440, would maintain the current estate tax levels through 2013.

All three lawmakers represent agricultural states, and Republicans have argued that the estate tax unfairly affects family farms that are passed down from generation to generation. McCaskill agreed with that sentiment.

“Our farmers and ranchers need more certainty right now, not less, which is why I'm leading this effort to prevent a hike in the estate tax and instead, lock in current levels," McCaskill said. “I'm going to keep fighting in every way I know how to provide certainty to our farm and ranch families so they can plan for the future."

McCaskill and Tester are in tough reelection fights this year, while Pryor will be up for reelection in 2014.