NEXT STEP: PASSING THE TAX DEAL…. Striking a deal on tax policy was difficult. Passing the deal isn’t much easier.

As should be obvious by now, the agreement has quite a few critics on the left, and congressional Democrats aren’t exactly thrilled with the end product.

A bloc of Senate Democrats could decide not to support a White House-brokered deal on tax cuts and unemployment benefits, thereby putting the plan in danger, according to the second-ranking Senate Democrat.

Senate Majority Whip Dick Durbin (Ill.) suggested Monday evening that many caucus members could threaten to back away from the deal as leverage to ensure it gets what it wants from a potential compromise with Republicans.

The House Democratic caucus has even more progressives in it, creating a challenge in that chamber, too.

And as it turns out, House Republicans will need plenty of convincing, too.

Some rank-and-file Republican lawmakers are uneasy about the tax cut deal their leaders struck with the White House.

Hours before the bipartisan agreement was announced on Monday, a few GOP members sent signals they weren’t comfortable with where the talks were headed.

Rep. Michele Bachmann (R-Minn.), in particular, said she and her allies would likely balk at any agreement that extends aid to those who’ve lost their jobs.

Most seem to expect the agreement to pass, but I think it’s a stretch to consider it a sure thing, especially if the most liberal and most conservative members balk. To get rank-and-file Republicans on board, GOP leaders will have to do some selling. To pick up Democratic support, the White House has a sales job in mind, and the process will begin today.

[A] number of Democratic aides said it will be imperative for Obama to personally sell Members on why the agreement he cut in behind-the-scenes talks with GOP leaders is the best way forward.

Aides expect that Obama will have to make a personal appearance at one or more caucus meetings on both sides of the Capitol prior to a vote on the tax package.

“He’s going to have to make the case,” one Democratic aide said.

Vice President Joseph Biden will make an appearance at Tuesday’s Senate Democratic policy lunch to begin that process.

I think pundits are probably making a mistake by assuming passage is a slam-dunk, but if I had to guess, I’d say it’ll squeak by. Congressional debate will likely get underway on Thursday.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.