Amid the initial elation from immigration advocates over a new proposal to overhaul U.S. border control laws was a sense of unease over the 844-page bill's core provision: a path to citizenship for immigrants living in the country illegally.

Proponents of immigration reform have pushed for decades to allow undocumented immigrants to become legal residents and, ultimately, citizens. But advocates expressed concern Wednesday that the path created by an eight-member bipartisan Senate group is too onerous, expensive and uncertain.

The fear, the advocates said, is that hundreds of thousands — or even millions — of the estimated 11 million undocumented immigrants living in the United States will never have the chance to remain in the country legally. Provisions in the bill could make it nearly impossible for "many immigrants to make it to the finish line and become citizens," said Marielena Hincapie, executive director of the National Immigration Law Center.