Democratic Sen. Ben Nelson (Neb.) said he isn't worried about blowback from activists if he votes to extend the Bush-era tax cuts because he didn't make "any campaign promises" about it.

Nelson indicated he would support a bill brokered with Republicans that extends the tax rates for all income brackets. On Saturday, he voted against the Democratic proposal that only kept the cuts for the middle class.

"I've made it clear, I didn't make any campaign promises about this, so I'm only focused on my end," Nelson told reporters before joining his Senate colleagues for their weekly lunch at the Capitol. "I've said that I thought that the debate shouldn't be about whether [the tax cuts] go to everybody, but for how long. And it looks to me like that that's the approach that the president and the negotiators have taken. Now, not everybody’s going to be happy with that.”

Nelson, who is up for reelection in 2012, has said on previous occasions he thinks the Bush-era tax cuts should be extended for all incomes. He noted Tuesday that voters want the parties to compromise.

"I think this last election made it clear, as previous elections have, that we have to do things in a bipartisan basis," said the two-term Democrat. "The closer the vote, the closer the ratio in the Senate."

Some liberal groups are outraged by what they perceived as the Democrats' willingness to cave on a multi-cycle campaign promise — to sunset the Bush-era tax rates on the highest income earners.

Adam Green, a leader of the Progressive Change Campaign Committee, said this vote could prompt primary challenges to Nelson and other Democrats.

"There's already an appetite for primarying someone like Ben Nelson, who is consistently scared of his own shadow and sides with corporations and special interests against his own constituents," said Green, whose group helped fund an unsuccessful primary challenge to Sen. Blanche Lincoln (D-Ark.) last cycle.

"Does voting to borrow billions of dollars to fund tax cuts for the rich stoke the fire even more? Sure. Voters will not forget this vote."

—This post was updated at 1:56 p.m.