U.S. Sen. John McCain (R-Ariz.) said Sunday that he didn't understand why some of his GOP colleagues have vowed to filibuster new gun control legislation, urging a full debate on the measures.

At least a dozen Republican Senators have backed a plan to filibuster new gun legislation, which was spearheaded by Sens. Rand Paul (R-Ky.), Ted Cruz (R-Texas), and Mike Lee (R-Utah). The Senate is expected to debate on some new measures, including an expansion of the nation's background check system, as early as this week.

McCain, who criticized Paul and Cruz as "wacko birds" last month, seemed baffled by their plan.

"I don't understand it," McCain said on CBS' "Face the Nation" Sunday. "The purpose of the United States Senate is to debate and to vote and to let the people know where we stand."

McCain said he would encourage Republicans to drop their threat of a filibuster.

"I would not only encourage it, I don’t understand it. What are we afraid of?" he said, noting that Senate Majority Leader Harry Reid has said he will allow amendments to be introduced to any bill.

"Why not take it up and amend it and debate? The American people would profit from it. I do not understand why United States Senators want to block debate when the Leader has said we can have amendments."

Watch McCain's comments below, via CBS: