By a vote of 15-13, the Senate Armed Services Committee approved an amendment by Sen. John McCain, R-AZ, that would require the President to deploy 6,000 National Guardsmen to the U.S. border with Mexico, but the Obama Administration has made it clear it will fight the provision to the end. The measure is now attached to a popular defense spending bill.

Committee Chairman Carl Levin, D-MI, told reporters Friday that he thinks the measure is unconstitutional, and said he received a letter from Gen. Jim Jones, Obama's National Security Adviser, "strongly opposing that language."

Levin predicted the McCain amendment would be quashed either on the Senate floor or in negotiations with the House over the defense bill.

"I believe that this would be an issue on the floor as to whether or not the Congress has the authority to direct troops to go to specific locations. There's no precedent for that, according to the Library of Congress. We can add, we can recommend, we can fund, we can prohibit, but can you direct the commander in chief to send troops to a specific place? There's no precedent for it," Levin said.

Levin added that "we'll be hearing from the President, I assume, or his security adviser again on this," predicting a tough fight on the Senate floor over whether or not to strike the provision from the bill.

Still, the bipartisan vote in committee on Thursday could make it difficult to strip the provision during floor debate, leaving what could be seen as a highly controversial decision to House-Senate negotiators.