Whether the estimated 11 million undocumented immigrants in the U.S. someday qualify for citizenship could depend on how lawmakers define "secure borders."

A bipartisan Senate proposal would hold off on setting a path toward citizenship until the government can certify it has met standards for border security.

Senators said the details have yet to be worked out. The plan includes better tracking of border exits to keep tabs on foreigners who enter the U.S. on visas and don't leave. By some estimates, these cases account for up to half of illegal immigrants.

The last effort to grant legal status to immigrants who are in the U.S. illegally, in 2006-07, was derailed by criticism that the federal government didn't have enough control over U.S. borders. Opponents painted a picture of millions of migrants crossing porous Southwestern borders at will.