Advocates for immigrants been been waiting for more than five years for Congress to take up a sweeping immigration overhaul, and it looks like that moment has finally arrived.

"This is tremendous momentum and an incredible step forward," says Greg Chen, director of advocacy for the American Immigration Lawyers Association, referring to a proposal from a bipartisan group of senators that includes a path to citizenship for undocumented immigrants.

"It does reflect a seismic shift in the debate," says Angela Kelley, vice president of immigration policy at the Center for American Progress, describing the plan's legalization provisions as "bolded and underlined."

Advocates describe the senators' framework as the biggest bipartisan breakthrough publicly released since 2007, when President George Bush's immigration overhaul died in Congress. Like Bush's plan, the senators' supports steps to legalization that are  "contingent upon securing our borders" and enforcing visa overstays, which must be accomplished before any undocumented immigrant receives a green card. Advocates say that's the most meaningful part of the plan but also the one that raises the most questions.

Their concern is that the conditions for gaining citizenship could wind up being so  stringent that undocumented immigrants could remain in legal limbo for the indeterminate future. "Is this citizenship in name only? If so, there is going to be some pretty dramatic backlash," says Mary Giovagnoli, director of the Immigration Policy Center.

The proposal, for example, calls for border security to "apprehend every unauthorized entrant," which has raised some eyebrows. "If that's going to be the standard, that's essentially an unrealistic, impossible standard to meet," says Chen.

Advocates are hopeful, however, that the senators' intent is to make the plan workable and not create an endless, open-ended timeframe for legalization. Bush's 2007 plan, for instance, set up similar triggers for gaining citizenship. According to Giovagnoli, experts estimated at the time that clearing out the backlog of visas would take about eight years. And this new blueprint, on its face, suggests that legislators might be willing to push a bill that's more generous than the 2007 plan.

"The 2007 bill was really conditioned on the idea that we can do legalization only by drastically reducing legal immigration in the future," with a point system that would have "radically reduced family-based immigration," says Giovagnoli. The new blueprint notably doesn't include such a stringent point system or any language suggesting that employment-based visas would only be increased at the expense of family-based visas, explains Chen.

Finally, advocates say, they're heartened to see that the framework is less punitive than Bush's 2007 bill, which Giovagnoli describes as "very heavily focused on interior enforcement measures, with an entire section enhancing penalties and deportation." The new proposal does call for bulked-up border security efforts — including an increase in "unmanned aerial vehicles," aka drones, to patrol the border — but it notably doesn't push for other kinds of enforcement measures. "It's more in keeping with the time and reflects an understanding that the debate today is different than the debate in 2007," says Kelley.

That omission partly reflects the battles being waged over immigration enforcement measures at the state level, most notably Arizona's SB 1070, which among other rules requires that police check the legal status of people they detain and suspect to be illegal immigrants.

That said, any number of provisions could be added to this blueprint if it does become the vehicle for a Senate bill, advocates acknowledge. But they are heartened by the fact that this Senate group is trying to get out ahead of the immigration debate, laying markers down before President Obama presses for changes in immigration law during a speech Tuesday in Las Vegas tomorrow.

"The senators wanting to go first I think shows how important everyone thinks this is," says Chen.