As the Washington Examiner has noted several times before, the key obstacle in getting a comprehensive immigration reform bill out of Congress is not the question of border security or legalizing existing immigrants. Lawmakers have reached a rough consensus on those issues.

The real stumbling block is whether the bill will include a guest worker program. The Chamber of Commerce wants one but unions hate the idea. Without both groups on board the coalition in Congress will likely fall apart.

The Washington Post has a report in today’s edition that the impasse has only gotten worse in the last week. It also points out one reason why the issue remains unresolved: The White House is refusing to take a side.

From the Post story:

Obama made no mention of a guest-worker program in a set of immigration principles that he laid out in a January speech in Las Vegas. The omission was notable, considering the bipartisan Senate group had included the idea in its principles that same week.

Instead, the White House has deferred to the Senate group, which includes four Democrats and four Republicans, to work out an agreement.

“If it’s included in line with the other principles that the president has rolled out in terms of what should be included in comprehensive immigration reform, that’s certainly something that we could support,” White House spokesman Josh Earnest said Wednesday of a guest-worker program. “But we’re going to reserve judgment on what that looks like until it’s actually produced.”

Administration officials say privately that the Senate group asked the White House to give the lawmakers “space” to take the lead in finding common ground between labor and business. Obama also is mindful of causing a political firestorm if he is seen trying to upstage the efforts of the senators, officials said.

But as the Post story notes, Obama also has a history of opposing guest-worker programs. As a senator, he backed a labor-supported poison pill amendment to the 2007 Kennedy/McCain immigration reform bill that stripped it of a guest-worker program. That helped to undermine Congress’ last serious effort at reform.

Thus, Obama is inclined to back the unions on this issue, but he also knows giving in to their demands will drive the Chamber of Commerce from the table. Without them they may not be able to win enough GOP support to pass the bill.

As a GOP aide notes in the Post article: “Everyone understands this is a critical piece for future flow . . .  Eventually, the White House will have to make a choice.”