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.

Reid has stitched together several proposals approved by the Senate Judiciary Committee earlier this month into one bill that will be debated once the Senate returns from an Easter recess on April 8. The bill will includes a Democratic plan to expand the nation's gun background check program; reauthorization of a Justice Department school safety grant program; and tougher new federal laws on gun trafficking.

The inclusion of Rubio -- a closely-watched, well-liked conservative, who like Paul is considering a 2016 presidential run -- should only add more exposure to Republican opposition to the legislation.

But unless and until the four Republican senators follow through on the threat, the letter is merely that -- a letter. And despite potential GOP opposition, Reid and Senate Democrats may still yet be able to muster enough votes to override a threatened filibuster.

Follow Ed O'Keefe on Twitter: @edatpost