Kentucky Sen. Rand Paulâs threat to filibuster any new gun restrictions is gathering steam, as a dozen of his Republican colleagues have now signed onto his plan.

The Kentucky Republican and Sens. Ted Cruz (R-Texas) and Mike Lee (R-Utah) first wrote to Senate Majority Leader Harry Reid late last month to warn him of their intention to try to tie up the Senate if, as planned, Reid moved forward with legislation that would expand background checks and attempt to crack down on interstate gun trafficking.

Reid is expected to bring a gun-control bill to the floor as early as next week, or perhaps the following week, and Paul is renewing his vow to try to block the measure. Paulâs follow-up letter, obtained by POLITICO, bears Mondayâs date and is signed by 13 Republicans, including fellow potential 2016 presidential aspirant Marco Rubio (R-Fla.) â who signed on shortly after Paulâs first threat was issued â and National Republican Senatorial Committee Chairman Jerry Moran of Kansas.

While Paul gained a new measure of fame among libertarian-minded voters on the right and left during a recent filibuster sparked by the administrationâs policy of targeted drone strikes, Reid has an ace in the hole. A new Senate rule would allow him to circumvent a filibuster on the motion to proceed to the gun bill by promising each party two amendments on the legislation. Under that scenario, Paul and his allies would still get a chance to raise their objections on the floor for hours on end, but they couldnât stop the Senate from starting debate on the bill.

Reidâs aides have discussed that option, but they havenât yet said that theyâll use it. Reid may be able to break a filibuster with bipartisan support, which would obviate the need to do an end run around Paulâs group. Sen. Lindsey Graham (R-S.C.) has said he wonât support a filibuster if Reid promises to allow for amendments on the floor.

In the letter due to be sent Monday, a carbon copy of the first missive, Paul and his expanded group of allies reiterate that they âintend to oppose any legislation that would infringe on the American peopleâs constitutional right to bear arms, or their ability to exercise this right without being subjected to government surveillance.â

The Gun Owners of America, a small group that has risen in influence because of its strict adherence to a pro-gun line, has pressured senators and the National Rifle Association to back Paul, Cruz and Lee.

âIf, you are an NRA member, contact them,â GOA wrote in an action alert sent to its own members, some of whom are also in the NRA, on April 1. âUrge them to join with us in supporting the Paul-Cruz-Lee filibuster. That means they should tell senators to oppose the motion to proceed to any gun control vehicle, and to oppose cloture on the motion to proceed to any gun control vehicle.â

In addition to Paul, Lee, Cruz, Rubio and Moran, the Republican who have signed the second letter are Jim Inhofe of Oklahoma, Richard Burr of North Carolina, Ron Johnson of Wisconsin, Mike Enzi of Wyoming, Jim Risch and Mike Crapo of Idaho, Dan Coats of Indiana and Pat Roberts of Kansas.

âWe will oppose the motion to proceed on any legislation that will serve as a vehicle for any additional gun restrictions,â they write.