(CNN) - Joe Biden is quickly becoming a powerful ally for Barack Obama in the Senate.
The Delaware Democrat, who is rumored to be on Obama's shortlist for VP, came to the Illinois senator's defense Thursday over charges Obama has not adequately addressed Afghanistan as chairman of a Senate Foreign Relations subcommittee.
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Obama is chairman of the Subcommittee on European Affairs - a panel that has jurisdiction over NATO's operations, including its efforts in Afghanistan. The McCain campaign, like Hillary Clinton's, has noted Obama has yet to hold a hearing dealing with Afghanistan since he became chairman nearly two years ago. South Carolina Sen. Jim DeMint, the ranking member on that committee and an ardent supporter of John McCain, wrote a formal letter earlier this week to Obama criticizing his failure to hold hearings - a move seemingly designed to undercut the Democratic presidential candidate's foreign policy credentials before he embarks on a trip abroad.
But Biden, conveniently the Chairman of the full Senate Foreign Relations Committee, hit back at DeMint Thursday, writing in a letter to him that NATO's mission in Afghanistan has been dealt with at the full committee level.
"Under my chairmanship the Foreign Relations Committee has addressed most Afghanistan issues at the Full Committee level," Biden wrote. "I believe that this is the best way of ensuring the most comprehensive examination of the complex issues involved, and of ensuring the highest-level Administration participation."
The move is likely to increase speculation Biden is being considered for the VP role, or another high-ranking post in an Obama administration. The veteran senator is considered an authority on foreign policy issues - an area where many voters have given the edge to McCain over Obama. Biden made news earlier this month when he acknowledged he would accept the VP position if it was offered.
Biden said the full committee has held three hearings over the last two years directly dealing with NATO and Afghanistan. But the Republican National Committee quickly noted Obama missed two of those hearings, and said Biden is "attempting to refine Obama's empty record on Afghanistan." (The RNC also noted Biden called Obama a "Johnny come lately" on the issue of Afghanistan last August when he was battling for the Democratic nomination.)
"Senator Obama has displayed great leadership on this issue: he called nearly a year ago for the deployment of at least two additional combat brigades to Afghanistan - it has since become the accepted position of a wide range of U.S. military officials, including the Chairman of the Joint Chiefs of Staff," Biden added.
Biden's defense comes at a key time for Obama - the Illinois senator heads to Afghanistan and Iraq soon, and has battled criticism from the McCain campaign over the last several days that he lacks an understanding of national security issues.