WASHINGTON (AP) — A bipartisan group of senators working to craft immigration legislation is focusing on how to define when the border is secure, one of several contentious issues that could cause the whole deal to collapse, a key Senate negotiator said Thursday.

Sen. Chuck Schumer, D-N.Y., pointed to "serious challenges ahead" as the lawmakers delve into the nitty gritty of border security, how to define a path to citizenship for illegal immigrants, and other issues such as a guest worker program — something business wants and organized labor has concerns about.

"Make no mistake about it, these are difficult and thorny issues, and all three of us have seen any one of these issues bring previous immigration bills down," Schumer said at a news conference with Senate Majority Leader Harry Reid and Sen. Dick Durbin, D-Ill.

Schumer, Durbin and the other six senators who proposed an immigration bill blueprint this week want assurances on border security before a path to citizenship can begin. President Barack Obama does not endorse such a linkage in his own immigration proposal, and the White House argues that the border is more secure now than it ever has been. But Republicans in the Senate group, including John McCain of Arizona and Marco Rubio of Florida, say they can't support an immigration bill that doesn't make a pathway to citizenship conditional on a secure border.

But just how to define a secure border remains to be determined, and the issue is front and center as the Senate group meets to draft legislative language.

Schumer and Durbin said they envision specific metrics that would need to be met. They didn't provide examples, but such measures could include crime statistics, arrests at the border, and the number of border agents deployed along the border.

"If we made the path to citizenship contingent on a safe and secure border, and just used that phrase, then it's in the eye of the beholder. It will always be subjective," Durbin said. "The idea behind a metric is to have something measurable, measurable, and we believe we can achieve that."

Rubio has said that "operational security" of the 2,000-mile border should be achieved before illegal immigrants can begin to achieve citizenship. He's defined that as law enforcement having a very high probability of being able to prevent somebody from illegally crossing the border or apprehending them if they do. A Government Accountability Office report in 2011 said that of the nine southwestern border sectors, only the Yuma, Ariz., sector had reported full operational security. Continued...