At the moment, a bipartisan group of senators is working through its fifth full day of “markup” on the immigration reform bill. As Yahoo News reports, they plan to address some of the more controversial elements of the bill, from including the number of visas for high-skilled immigrants, to the question of whether to allow people in same-sex marriages to apply for green cards for their spouses.

All of this, of course, has been lost in the obsession over scandals — both real and imagined. What’s also been lost is the extent to which the ongoing fight over immigration reform is dividing the Republican Party between its more pragmatic, pro-reform members — conservatives like Marco Rubio or Arizona’s Jeff Flake — and it’s more doctrinaire, anti-reform ones, like Alabama Senator Jeff Sessions. So far, the fight has favored the former. Conservative proponents of comprehensive immigration reform have the largest platform — on account of their role in crafting the Senate bill — and recently scored a victory after the Heritage Foundation fell on its face following the release of its study on the “costs” of reform, which was widely-panned, as well as connected to racist ideas about the intelligence of Hispanic immigrants.

But anti-immigration conservatives haven’t given up the fight. This morning, a coalition of 150 conservatives — which includes Rich Lowry of the National Review, Phyllis Schlafly of Eagle Forum, editor Eric Erickson, and former Florida Representative Alan West — issued a letter declaring their absolute opposition to the comprehensive immigration bill, and urging Senate Republicans to scrap the entire project.

“No matter how well-intentioned, the Schumer-Rubio bill suffers from fundamental design flaws that make it unsalvageable,” the letter says. “Many of us support various parts of the legislation, but the overall package is so unsatisfactory that the Senate would do better to start over from scratch.”

Their main complaints echo the line introduced by the Heritage Foundation. The Senate bill would keep the nation’s borders insecure, give “excessive control” to the Obama administration on immigration law, and “bankrupt” retirement programs by introducing a huge number of new recipients.

It’s hard to find a factual basis for each of these objections. The border with Mexico is currently more secure than its ever been, and the additional returns to adding even more security — on top of what is already present in the bill — are likely to be low. Likewise, there’s no way to avoid additional control for the administration — immigration is handled by federal agencies, which are managed by the White House. And more than a few observers have addressed the claim that immigration would bankrupt retirement programs; the assertion relies on the untenable assumption that every immigrant would receive legal status, become a citizen, and fail to contribute taxes or productivity.

Of course, the factual claims of the letter are irrelevant. What matters is that a key part of the Republican Party has announced its categorical opposition to comprehensive immigration reform, which diminishes its chance for passage. Unless supporters can get overwhelming support in the Senate — enough to break a filibuster, and then some — then it’s unlikely they’ll force John Boehner to act. And if he does, he’ll still have to deal with the consequences of an unhappy conservative opposition.

In other words, there’s no reason to think the effort is necessarily doomed, but there’s no reason to be optimistic either.

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