Two groups normally allied with Republicans have bolted from the party’s effort to ban judicial filibusters — the first major defections from a conservative push to prevent Senate Democrats from blocking President Bush’s judicial nominees.

The National Right to Work Committee, a 2.2 million-member group critical of unions, and the Gun Owners of America, with 300,000 members, say they fear eliminating judicial filibusters could eventually lead to doing away with filibusters altogether.