To boost the population of low-skilled foreign workers in the United States, congressional leaders included in the recent 2,000-page omnibus spending bill an unrelated policy rider sought by the business lobby.

Who put the immigration measure in the must-pass year-end spending bill that was negotiated in secret? That's still a mystery. But here's the story as we know it.

Among the dozens of visa categories that allow foreigners to enter the United States is the H-2B visa. This is a "non-immigrant" visa, and is called a "guest-worker" visa. While the H-1B visa is for high-skilled workers, the H-2B visa is for low-skilled worker. Under current law, the U.S. State Department can give out 66,000 of these visas per year.

Guest workers are allowed to enter only for a few months, and they are allowed to stay in the country only as long as they stay employed. This program is a priority of the business lobby, and it upsets blue-collar workers and labor unions.

You can't blame working-class voters for disliking this program. If you're out of work or tenuously employed, or if your wages have been stagnant or down, then the last thing you want is the federal government importing more competition.

Government officials and industry executives say that these guest workers only take jobs Americans won't do, and regulations on the program are aimed at ensuring this. (These requirements are easy to evade, however.) Given the U.S. welfare state and American attitudes towards certain jobs, finding domestic labor — especially seasonal labor — at a price that still allows profits is nearly impossible for some businesses.

We also know that historically, some employers have favored guest-worker programs because they gave them added leverage over employees. One sugar grower explained to the U.S. Department of Agriculture: "The vast difference between the Bahama Island labor and domestic, including Puerto Rican, is that labor transported from the Bahama Islands can be deported and sent home, if it does not work, which cannot be done in the instance of labor from domestic United States or Puerto Rico."

in other words, workers won't complain much if they know firing equals deportation.

Back in July, the Appropriations Committee passed a Homeland Security funding bill that included a provision effectively lifting the cap on the number of temporary guest workers. In brief, the measure would exempt from next year's 66,000-worker cap any of this year's guest workers who return next year — and so on for the next few of years.

Theoretically, this change could quadruple the number of low-skilled temporary guest workers in the country — which is what opponents cried. More likely, the change will cause a small increase. Aides to House Speaker Paul Ryan say they expect about 8,000 additional visas. In 2007, however, nearly 70,000 returning guest workers took advantage of a similar provision, according to State Department data.

Increasing foreign workers through an appropriations bill is a bit odd. The Judiciary Committee has jurisdiction over immigration law, but a committee aide told me Chairman Bob Goodlatte wasn't responsible for the provision in the appropriations bill. Appropriations aides did not respond to my queries on Tuesday.

The clearest fingerprints on the measure: Rep. Charles Boustany, R-La., in June had introduced a standalone bill to permanently exempt returning guest workers from the H-2B cap.

Expanding guest workers was a priority for the business lobby. The U.S. Chamber of Commerce included in a list of its 2015 priorities "secure additional essential workers." The Chamber and the National Fisheries Institute both reported this year that they lobbied on a "returning worker exemption" for the H-2B program. The fishing industry is an ally to Boustany, who represents Cajun country including most of Louisiana's Gulf coast.

The House Appropriations Committee passed the Homeland Security bill in July, including the H-2B rider. The committee published a summary of the bill, which never mentioned the H-2B expansion.

The Senate bill, which had passed in June, didn't include the rider. Neither bill ever moved to the floor of either chamber, as happened with a handful of other appropriations.

In the wee hours of Tuesday morning, Dec. 15, congressional leaders released the 2,000 pages of of the omnibus bill. One Republican member told me that only then did he learn the H2B provision was in the bill. Sen. Jeff Sessions had earlier heard rumors that the House-Senate negotiators would stick the provision in, and he had written a letter of protest to Republican leadership.

After the bill was made public, Majority Whip Steve Scalise, also from Louisiana, touted the H-2B provision as a solution to "the current and projected H2B worker shortage." Scalise credited the Judiciary Committee.

It's easy to see why the Chamber would want more cheap labor. It's easy to see why the fishing industry, Scalise and Boustany would push the legislation. But why would Republican leaders include this divisive provision in the year-end must-pass omnibus?

Angry conservatives blame Ryan. Ryan deflects that blame, saying that he simply deferred to the House Appropriations Committee.

It's a clear fracture in the Republican base. On one side are working-class conservatives and a base skeptical of increased immigration. On the other side is the business lobby. Can Ryan figure out how to patch up that fracture?

Timothy P. Carney, The Washington Examiner's senior political columnist, can be contacted at tcarney@washingtonexaminer.com. His column appears Tuesday and Thursday nights on washingtonexaminer.com.