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.