Sen. Marco Rubio (R-Fla.). (Chip Somodevilla - Getty Images)

And then the trio became a quartet.

Sen. Marco Rubio (R-Fla.) is joining with three other Senate Republicans in threatening to filibuster the Senate Democrats’ gun violence legislation slated for consideration next month.

Rubio's office announced Thursday that he's joining with three other Senate conservatives — Rand Paul (R-Ky.), Ted Cruz (R-Texas) and Mike Lee (R-Utah) – in opposing any legislation that might infringe on the Second Amendment.

Paul, Cruz and Lee announced Tuesday that they had sent a letter to Senate Majority Leader Harry M. Reid (D-Nev.) threatening to filibuster Reid's gun violence legislation, leaving open the possibility of another marathon session mirroring Paul’s nearly 13-hour filibuster on the CIA’s drone policy.