The battle over border security started on the Senate floor Thursday with a defeat for Republicans looking to ensure that the southwest border with Mexico is locked down before any of the nation's 11 million unauthorized immigrants can gain legal status.

The Senate voted down an amendment proposed by Sen. Chuck Grassley, R-Iowa, requiring the Department of Homeland Security to prove "effective control" of the border for six months before unauthorized immigrants can get temporary legal status, the precursor to earning their green cards and U.S. citizenship.

The vote was 57-43 to strike down Grassley's amendment, and Republicans said it did not bode well for their efforts to enhance border security in the sweeping immigration bill they are debating.