Sens. Rand Paul, Ted Cruz and Mike Lee are threatening to filibuster gun-control legislation, according to a letter they plan to hand-deliver to Senate Majority Leader Harry Reid’s office on Tuesday.

“We will oppose the motion to proceed to any legislation that will serve as a vehicle for any additional gun restrictions,” the three conservatives wrote in a copy of the signed letter obtained by POLITICO.

Reid plans to bring up a gun-control measure that focuses on broadening background checks and cracking down on interstate gun-trafficking after the current Senate recess.

Conservatives are concerned that once that bill reaches the floor, amendments could stiffen restrictions on gun control.