The Bush administration and key senators are working on a deal that would delay a sweeping immigration overhaul until the border is fortified and leave illegal immigrants waiting up to 13 more years to gain legal status.

Officials familiar with the discussions say that despite a series of concessions by both Republicans and Democrats, a final agreement may not come before the Senate opens debate on the issue next week.

Still, the outlines of a possible deal have taken shape in almost daily secret talks involving two members of President Bush's Cabinet. As contemplated, the proposal would delay the process of giving legal status to undocumented immigrants and guest worker visas to new arrivals until the administration beefs up border security and implements a high-tech identification system for temporary workers. Such measures are expected to take up to two years.

Even then, officials said it would take up to 13 years — along with large fines and a trip to their home country — before the 12 million men, women and children estimated to be in the U.S. illegally could get permanent legal status, or green cards. The government would spend eight years clearing visa backlogs for immigrants currently waiting in line, and then spend about five years processing those here illegally.

Sen. Edward Kennedy, D-Mass., has been leading negotiations with Republican senators and White House officials in hopes of cutting a bipartisan deal on the issue before the Senate wades into an explosive immigration debate.

Absent such a compromise, Senate Majority Leader Harry Reid, D-Nev., is expected Wednesday to bring up a much more liberal measure approved last year, in preparation for a lengthy and wide-ranging debate on the matter starting next week.

Sen. Arlen Specter, R-Pa., one of the negotiators, made a plea Tuesday for more time to reach a bipartisan deal, warning that rushing a debate could provoke a filibuster. Specter called the emerging agreement a "grand bargain."

The political stakes for both parties are high. Democrats are eager to deliver a measure that appeals to their liberal base by allowing the huge undocumented population to earn legal status and welcoming new immigrants.

Republicans are seeking to neutralize an issue that party strategists believe hurt them in the 2006 elections. Some GOP lawmakers, however, are wary of alienating core conservative backers who want to crack down on immigration and enact stricter laws.

A group of House Republicans led by Rep. Lamar Smith of Texas, the senior Judiciary Committee member in his party, wrote to senators Tuesday decrying as "mass amnesty" any bill that grants legal status to undocumented immigrants instead of deporting them.

Reshaping the immigration system is a top priority for President Bush, who has said he hopes Congress can deliver a measure by the end of summer.

Weeks of negotiations have yielded tentative agreements within the bipartisan group on many divisive issues, but they have yet to agree to any package, and both sides face major risks.

Kennedy must balance his appetite for a deal against the concerns of liberal groups that find the conditions for immigration policy changes — sometimes referred to as "triggers" — punitive and unacceptable. They also object strenuously to proposed changes that would curtail legalized immigrants' ability to petition for relatives to come to the United States and would forbid temporary workers from bringing family if their income is below 150 percent of poverty level and they don't have health insurance.

Republicans, led by Sen. Jon Kyl, R-Ariz., are fearful of appearing to embrace an overly permissive measure.

"You just can't comprehensively reform something until you first solve the problem, and the problem is an insecure border. Once it's secure, then you can make your reform work," said Sen. Johnny Isakson, R-Ga., a leading proponent of the conditions.

The steeper obstacles for an immigration program come as the White House works to gain more Republican backing for an overhaul. The measure up for discussion in the bipartisan talks is tougher than one that the Senate passed a year ago with broad Democratic support but relatively few GOP votes.

"They're not dealbreakers for us, but the flexibility has to be good enough that I can go to our community and say, within the context of the policy, this is worthy enough of supporting — but not a deal at any cost," said Sen. Robert Menendez, D-N.J., who has been in on the talks.

In Miami's Little Havana on Tuesday, the Coalition for Comprehensive Immigration Reform launched a media campaign targeting Sen. Mel Martinez, R-Fla., the party chairman who supported last year's measure, for backing the Bush administration's new, more restrictive stance.

One ad urges Martinez to "remember your roots," and support a measure similar to the 2006 immigration bill, warning, "If you turn your back on this, a generation of Latinos will not forget."

Many lawmakers believe reaching a bipartisan immigration compromise could inoculate both parties against political peril in 2008.

"Immigration was used against those people who supported a bill last year, including me," Menendez said. "A bipartisan bill would be a lot more difficult to use as a political tool against people in either party."