Usually it's unions arguing that a guest worker program favors Big Business, but this time a Republican senator from Alabama is making the case.

Sen. Jeff Sessions, R-Ala., is not a Big Labor booster. He has a lifetime rating of just 11 percent on the AFL-CIO ranking of lawmakers' votes.

So it is more than a little ironic when he suddenly starts echoing the criticisms unions have made regarding the Gang of Eight's immigration reform bill.

The bigger irony is that Sessions is actually making the case more ardently than Big Labor itself. Overall, unions are backing immigration reform. You will hardly find a better example of the tangled politics and strange bedfellows involved in the current immigration debate.

To understand what is going on, it helps to look back to the last major immigration reform push, the 2007 Kennedy-McCain bill. That was undermined in part by a war between the Chamber of Commerce and the AFL-CIO over guest worker programs.

Big Labor succeeded in getting Senate Democrats to adopt an amendment by long-time immigration critic Sen. Byron Dorgan, D-N.D., stripping the bill of its guest worker program.

The easily foreseeable result of this was the Chamber pulling its support, undermining the bill's already-fragile GOP backing. (The backers of this poison pill amendment included then-Sen. Barack Obama, D-Ill.)

The Chamber and the AFL-CIO have made a point of trying to prevent a replay of that this time around. They struck a deal earlier this year on guest worker programs that allowed them but attached various strings.

For high-tech visas, for example, the agreement required that businesses prove that American workers were not already available first before visas were issued.

Not everyone was okay with that. The high-tech industry lobbied hard in committee to loosen those requirements, straining the Big Labor/Big Business immigration alliance.

The current bill increases those high-tech visas from 65,000 annually to at least 110,000 and potentially as high as 180,000. Some unions say as high as 230,000. You need a diploma from MIT just to understand the series of mathematical triggers in the bill that determine the number.

Unions that represent the high-tech sector are not happy with the changes. Communications Workers of America President Larry Cohen said in statement the Senate bill "will allow preferential treatment by corporations for foreign workers at the expense of U.S. workers."

The International Federation of Professional and Technical Engineers has urged senators to vote against the bill. The real reason Silicon Valley wants foreign workers, it argues, is that they are cheaper.

Unions cite a study by the Economic Policy Institute that claims there is no shortage of high-tech workers. The U.S. colleges already graduate more people with high-tech degrees than there are job openings.

EPI isn't exactly unbiased here. Nine of liberal nonprofit's boardmembers are union leaders, including CWA's Cohen. But the conservative Center for Immigration Studies has produced similar research. (Then, again it opposes immigration too.)

But while IFPTE opposes the bill, CWA does not. "We will try to gain improvements in (high-tech visas), but overall we support reform. There will be compromises on both sides," CWA spokesman Candice Johnson told the Washington Examiner.

Last week, the AFL-CIO even announced an immigration lobbying blitz. "The labor movement is committed to putting its full weight behind the cause of citizenship," President Richard Trumka said. Translation: Doubling or even tripling the number of immigrant high-tech visas is bad but not that bad.

Big Labor is willing to swallow it because they've gotten much better at organizing immigrant workers in recent decades. With unionization down to just 11.3 percent of the workforce, legalizing 11 million immigrants would give Big Labor a chance to grow again.

That's left it up to Republicans like Sessions — who simply opposes immigration, period — to make Big Labor's case against it.

"There is no doubt that this legislation will hurt struggling U.S. workers — immigrant and native-born, union and nonunion, poor and middle class — a fact about which too many politicians and labor leaders have been much too silent," Sessions said recently.