Senators on Wednesday approved a plan by a broad bipartisan margin to bolster security along the U.S.-Mexico border, a key concession to Republicans who plan to support the overall immigration measure.

Sen. John Hoeven (R-N.D.). (J. Scott Applewhite/AP)

By a vote of 69 to 29, senators amended the immigration bill to include provisions that would deploy roughly 40,000 U.S. Border Patrol agents along the Mexican border, require construction of 700 miles of fencing along the southern border and authorize the use of new radar and unmanned aerial drones to track illegal border crossings.

"Americans want immigration reform, but they want border security first," said Sen. John Hoeven (R-N.D.), a key backer of the amendment who helped secure enough GOP votes.But a majority of Senate Republicans voted against the proposal, saying it is a costly and ineffective way to ensure that people stop crossing into the United States illegally.

The agreement "throws money at the border, but it doesn't get the job done," said Sen. Charles E. Grassley (R-Iowa.), who added that the overall bill stresses "Legalization first, border security later."

All members of the Senate Democratic caucus voted for the amendment, as did 15 Senate Republicans; 29 Republicans voted against the amendment.

Supporters of the immigration bill are hoping to hold a vote on final passage as early as Thursday evening, but most Republicans object to holding votes on more amendments that would make minor changes to the legislation.

Senate Majority Leader Harry M. Reid (D-Nev.) said again Wednesday that talks on how to proceed are going "backwards."

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