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. Paul’s follow-up letter, obtained by POLITICO, bears Monday’s date and is signed by 13 Republicans, including fellow potential 2016 presidential aspirant Marco Rubio (R-Fla.) — who signed on shortly after Paul’s first threat was issued — 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 administration’s 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 couldn’t stop the Senate from starting debate on the bill.

Reid’s aides have discussed that option, but they haven’t yet said that they’ll use it.