Writing for ABC Univision, Jordan Fabian notes that it’s not just Republicans who might oppose and vote against comprehensive immigration reform. In the Senate, at least, there are also Democrats who also have concerns that might lead them to oppose a bill. He names four lawmakers. First is Vermont Senator Bernie Sanders, who voted against the 2007 comprehensive immigration reform bill over opposition to its guest worker program. Even now, notes Fabian, Sanders is weary of bringing in large numbers of foreign workers:

“I’m very dubious about the need to bring foreign unskilled labor into this country,” Sanders, who caucuses with the Democrats, told The Washington Post last month. “These are kids, young high school graduates, and the unemployment rate is just extremely high. I do not understand why they cannot hire those people and need foreign labor.”

Sanders wasn’t the only senator to vote against the 2007 bill. Montana Senator Max Baucus and Arkansas Senator Mark Pryor also opposed the Republican-crafted proposal. Like Sanders, Pryor is concerned about the labor implications of immigration reform — he’s not sure if the United States should be increasing its number of high-skilled visas. As for Baucus, he opposed a path to citizenship in 2007, and it’s not clear if he’s reversed that position.

There’s also Joe Donnelly, the new Indiana senator whose 2010 congressional campaign included a promise to “deport illegals who commit felonies and eliminate amnesty, because no one should ever be rewarded for breaking the law.” He even voted against the DREAM Act, rejecting what seems to be a consensus position in the Democratic Party.

The advantage of comprehensive immigration reform is that it satisfies a wide variety of concerns. If you don’t like one provision — the guest worker program, for instance — then you can sign on to the path to citizenship, or something else. If supporters of immigration reform have been unified on anything, it’s opposition to the piecemeal approach floated by some House Republicans, in which immigration reform would be broken up and passed in pieces. The problem with this is that as soon as you disentangle each provision from the other, the coalition for passage falls apart. Keeping reform in one comprehensive package increases its chances of success.

But we shouldn’t assume unanimous Democratic support for an immigration bill. If comprehensive reform becomes unpopular as the debate over it unfolds and the public focuses on it, you could also see retreat from red state and vulnerable Democrats. That prospect would become even more likely if all Republicans, even pro-reform ones in the Senate, end up turning against the proposal, which is still in the realm of possibility, if the Senate debate goes in a bad direction. All of which is to say that the chances of reform passing remain very fragile indeed.

Jamelle Bouie is a staff writer at The American Prospect, where he writes a blog.