It’s looking pretty good lately for immigration reform. Big Business and Big Labor, represented by the U.S. Chamber of Commerce and the AFL-CIO, say they have reached a deal to regulate the flow and status of temporary work visas. Democrats and Republicans, represented by Sens. Charles Schumer of New York and Lindsey Graham of South Carolina, say they may have legislation as early as next week.

Or maybe not. Republican Sen. Marco Rubio of Florida, another member of the bipartisan “Gang of Eight” that has been working on immigration reform, called reports of progress premature. Rubio is at the center of the action on comprehensive immigration reform for one reason only: He placed himself there. But unlike Graham or Schumer — or, for that matter, the consistently anti-reform Republican from Alabama, Sen. Jeff Sessions — it’s not entirely clear from day to day whose interests Rubio represents.

If Rubio’s presidential ambitions are genuine, he surely has more on his mind than his Florida constituents. The fear is that Rubio will use his pivotal position in negotiations not to advance immigration reform but to win the loyalty of conservatives who are implacably opposed to it. Despite the progress made so far, opportunities for mischief abound. Three major obstacles remain to viable legislation.

The first is temporary work visas. The two sides appear to have an agreement in principle, but mutual suspicions remain. The interests of labor and capital are simply not well aligned on this issue. The AFL-CIO, especially its construction trades, fears that businesses want to import a flood of cheap labor to drive down wages. The Chamber of Commerce fears that labor wants to strangle work visas in red tape, leaving businesses unable to find workers at reasonable rates.

Second is the fate of the estimated 11 million undocumented immigrants already in the country. There has been no softening against “amnesty” in some quarters. Some hardliners oppose a path to citizenship under any circumstances. Others want to place so many obstacles — years as resident noncitizens, fines, back taxes, etc. — in the path that they would transform amnesty into purgatory. The United States can’t create an official noncitizen underclass without doing permanent damage to both its ideals and its identity.

Third is the issue of border security. One of the more pernicious notions circulating Washington is a proposal to make citizenship for undocumented immigrants contingent on reaching certain benchmarks in border control. In effect, 11 million people would be held hostage to circumstances beyond their control. What’s more, the reliability of the benchmark data — illegal border crossers don’t generally submit to a census in transit — would be as dubious as the incentives for cooking the numbers are vast. Sometimes very bad ideas enjoy very broad support. This is one.

Opponents of immigration reform will use each of these vulnerabilities to try to bring the legislation crashing down. They may yet succeed. Even modest legislative encumbrances in the Senate could be sufficient to scuttle reform altogether in the House, where it will need significant momentum to overcome the Republican majority.

All of which is to say that supporters of an immigration overhaul can’t afford to relax. Their opponents make up in intensity what they lack in numbers. Despite the jovial mood on the Sunday talk shows, Rubio is right: Reform still faces real enemies, real obstacles and, as a result, potentially fatal delays. A march scheduled for April 10 in Washington is a good opportunity for reformers to show strength in numbers. But while 90 percent of life may be just showing up, passing difficult legislation requires considerably more.

Bloomberg News (April 3)