Supporters of comprehensive gun control legislation are ratcheting up  pressure on wavering lawmakers, trying to clear the way for Senate Democratic  Leader Harry Reid to proceed with a bill in the face of mounting roadblocks from  conservative members. 

At least 14 Republican senators have threatened to filibuster the firearms  legislation, complaining that it would curb the rights of the law-abiding while  doing little to reduce crime. "The more people learn about the consequences of  current gun control proposals, the less they will support the new restrictions,"  Sen. Mike Lee, R-Utah, said. 

If Lee and his colleagues succeed, their objections would require any bill to  garner 60 votes, a heavy lift for Reid and the White House. 

But as negotiations continue on a compromise measure that could sway moderate  hold-outs, powerful advocacy groups are rolling out new ads and campaigns to win  them over.