Updated: 10:35 p.m.

Sen. Bernie Sanders on Monday all but promised to filibuster President Barack Obama’s controversial agreement with Republicans to extend all of the Bush-era tax cuts for two years.

Appearing on “The Ed Schultz Show” on MSNBC, the Vermont Independent lambasted the agreement, which also includes a 13-month extension of unemployment insurance, a two-year exemption from the estate tax for estates worth up to $5 million and a host of provisions from last year’s stimulus bill.

“I think it is an absolute disaster and an insult to the vast majority of the American people,” Sanders told Schultz, adding that Democrats opposed to the deal “are right. We’re talking about social justice. They’re talking about more tax breaks for billionaires who don’t need it.”

While Sanders did not explicitly say he would filibuster the bill, he made his intention clear, saying, “I’ve got to tell you, I will do whatever I can to see that 60 votes are not acquired to pass this piece of legislation.”

Whether Sanders can be persuaded to not try to block the bill remains unclear: Vice President Joseph Biden is scheduled to attend Senate Democrats’ luncheon Tuesday to defend the agreement, which Obama cut in secret talks with the GOP.

Senior Democratic aides said that while the deal — particularly the estate tax provisions — would be a bitter pill for the caucus to swallow, the agreement included enough pieces of the stimulus bill to make it worthwhile.

But Sanders, for one, said Monday that he believes Democrats should stand up to the GOP and the White House in the tax debate. “I think we’ve got to hold tough on this, hold firm on this, and not concede to Republicans, who, as you indicated, have absolutely no inclination to compromise,” he told Schultz.

Other Democrats, while not going so far as Sanders in threatening a potential filibuster, were unwilling to immediately throw their weight behind the bill.

“I need to look at it. I’m very unhappy about it,” said Sen. Sherrod Brown (D-Ohio) when asked on CNN whether he would vote for the deal. When pressed if he would stand on principle and vote “no,” Brown said, “if I think this deal is as serious as a problem as it looks at this point, I’m likely to do that.”