MOMENTUM FOR a sweeping deal on immigration reform is building so fast that it can be difficult to remember that scores of congressional Republicans, especially in the House, have not budged from the what-part-of-illegal-don’t-you-understand orthodoxy that helped cost the party the presidency. Fearing nothing but a primary challenge in their extravagantly gerrymandered districts, House backbenchers have little incentive to switch positions and embrace any pact that includes what they regard as amnesty.

But Republicans with broader constituencies and national aspirations — hello, Sen. Marco Rubio of Florida — were helped to see the light by the spanking dealt to the party by Latino voters last November. They know that unless the GOP evolves, its long-term prospects are dim.

Hence Mr. Rubio’s public wavering on an immigration accord. A member of a bipartisan group of eight senators trying to hammer out the details of an immigration bill, Mr. Rubio appears paralyzed — or to be trying to have it both ways. At first he led the charge, trying to brand an overhaul of the immigration system as his signature achievement. Then, when progress was made last weekend, he backed away, warning that talk of a breakthrough was premature.

Is the Florida senator the once and future darling of the tea party, throwing bombs from the sidelines? Or is he a substantive architect of a workable new system? It’s one or the other; Mr. Rubio needs to decide.

He’s been handed the opportunity thanks in part to a critical agreement between influential business and labor groups on future immigration — among the most contentious of issues that helped kill past attempts at reform.

After weeks of hard bargaining, leaders of the U.S. Chamber of Commerce and the AFL-CIO struck a compromise that would allow issuance of up to 200,000 visas annually for low-skilled workers in hotels, restaurants, construction sites, janitorial positions and the like. The workers would be treated fairly — free to switch jobs; paid prevailing wages so as not to undercut American workers’ pay; and eligible to apply for permanent immigration status. That’s a good and humane deal.

But some terms of the business-labor truce are troubling. It would start too slowly, with just 20,000 visas in the first year (possibly 2015), which might be far too little if the economy continues to accelerate. It would create a new government agency that would certify labor shortages in particular industries and authorize visas to fill job openings — a system that smacks of centralized planning. It would have been much wiser to allow the labor market to determine its own needs.

Still, the pact represents a political breakthrough. Congressional negotiators seem to be close to agreement on other aspects of a bill — border security; verification of employment status; and, most important, a pathway to citizenship for 11 million undocumented immigrants.

If GOP stars such as Mr. Rubio decide once and for all to lead, that may be enough to sway fence-sitting Republicans in the House. If they waver, this year’s attempt at immigration reform, like those of past years, is as good as dead.