Given the disdain some conservatives reserve for Republicans who consort publicly with Democrats, he had reason to be. The next time Mr. Rubio is likely to appear with his colleagues in the eight-person bipartisan group could be an even bigger moment, when its members officially introduce joint immigration legislation this month. The probable tableau seems ready-made for problems in the 2016 Republican presidential primary fight in which many expect Mr. Rubio to partake: images of Mr. Rubio, smiling and celebrating alongside Democratic senators and maverick Republicans as he claims co-authorship of an overhaul of the nation’s immigration laws that many Republicans will reject. And so the question percolating on Capitol Hill has become: Will Mr. Rubio, an up-and-coming young conservative elected on a 2010 Tea Party wave, ultimately sign onto the immigration bill that he has been helping to draft ever since the November election? “We have to see if the Boy Wonder plays ball or not,” said Frank Sharry, executive director of America’s Voice, a pro-immigration group. For now, the answer — among members of the bipartisan group, immigration watchers and even Mr. Rubio’s own staff — is a tentative yes, even with Mr. Rubio increasingly urging caution about racing ahead with any immigration measure as the unveiling draws nigh. “We understand Marco is not going to be rushed into anything,” said Senator Charles E. Schumer, Democrat of New York and a member of the group. “But we don’t doubt his commitment to seeing this through at all.” In recent days, Mr. Rubio has begun to sound nervous again when it comes to the immigration legislation. He sent a letter to Senator Patrick J. Leahy, the Vermont Democrat who is chairman of the Judiciary Committee, counseling against “excessive haste” in changing immigration law. On Sunday, just moments before two of his fellow bipartisan group members were set to appear on NBC’s “Meet the Press,” he released a statement that warned, “No final agreement on immigration legislation yet.” Mr. Rubio finds himself in an extremely delicate position as a rising conservative hero and a proponent of immigration law changes. As much as the fate of the bipartisan group’s legislation hangs on Mr. Rubio, who is perhaps the only member of the group with the conservative credentials to sell the plan to skeptical voters, the fate of Mr. Rubio’s ambitions for higher office are also inextricably bound up with the immigration legislation and his base’s reaction to it. So far, Mr. Rubio has seen early success wooing grass-roots Republican voters. When he joined the bipartisan group late last year, after Mitt Romney lost the presidential election in part because he was overwhelmingly rejected by Hispanic voters, Mr. Rubio quickly went on a one-man blitz of conservative news media outlets, explaining his guiding immigration principles and winning plaudits from the likes of Rush Limbaugh and Sean Hannity. “The argument is that it’s not conservative policy to have 11 million illegal immigrants in this country,” said Phil Musser, a Republican consultant and former executive director of the Republican Governors Association. “By showing up and by boldly deciding to engage people who have had their fingers in their ears on this issue, he’s been able to decalcify a process that’s been stuck for a long time.” He and his staff have studied the mistakes of the last attempt at immigration overhaul, in 2007, and have tailored their pitch to address conservative concerns. Mr. Rubio stresses that strict goals for border security must be met before any illegal immigrants can be put on a path to citizenship. And he talks of refining the current system — from increased enforcement to a workable plan for future legal immigrants — saying he wants to ensure the country does not face another wave of illegal immigrants down the road. “His emphasis on strong border security and enforcement is a big deal, and I think the Democrats are finally beginning to get that, and I think that is because of his strong leadership and communication skills,” said Mel Martinez, a former Republican senator from Florida who was part of the failed 2007 attempt at an immigration overhaul.
