As talks progressed Wednesday on a compromise border security amendment, Senate negotiators appeared to have hit a crucial sticking point that, if not resolved, could undercut the viability of the “Gang of Eight” immigration-reform package.

Republicans who could support the overall bill but who are unhappy with its current border security provisions want Congress to spell out an enforcement strategy that the White House would have to follow. Failure to follow Congress’ mandate could delay the legislation’s plan to provide a pathway to citizenship for 11 million illegal immigrants.