The Senate’s leading conservative trio -- Rand Paul (R-Ky.), Ted Cruz (R-Texas) and Mike Lee (R-Utah) – are threatening to filibuster the Senate Democrats' gun violence legislation slated for consideration next month, leaving open the possibility of marathon speeches in the mold of Paul’s nearly 13-hour filibuster on the CIA’s drone policy.

Sen. Rand Paul (R-KY) listens during a news conference with other Senate Republicans to announce a proposed balanced budget amendment to the Constitution in the U.S. Capitol on March 31, 2011 in Washington, DC. (Photo by Brendan Hoffman/Getty Images) Sen. Rand Paul (R-KY) (Photo by Brendan Hoffman/Getty Images)

The senators sent a brief letter to Majority Leader Harry M. Reid (D-Nev.) outlining their opposition to “any legislation that would infringe on the American people’s constitutional right to bear arms,” signaling they will oppose the procedural vote to begin debate on the gun package when the Senate returns from its current two-week spring break.

"We will oppose the motion to proceed to any legislation that will serve as a vehicle for any addition gun restrictions," the three wrote.

It's not clear yet whether the effort would be another talking filibuster, like the one Paul led against CIA director John Brennan's nomination last month, or a silent filibuster that would simply require Democrats to get 60 votes to proceed to an up-or-down vote.

Cruz spokeswoman Catherine Frazier says her boss "is prepared to use any procedural means necessary to prevent stricter gun control laws."

The letter can be found here.

Reid announced last week that he was cobbling together several of the proposals approved by the Senate Judiciary Committee earlier this month into one legislative package that will be debated on the floor for several weeks. Those proposals include a Democratic draft of a new universal background check on all legal gun purchases; school safety provisions; tougher new federal laws on gun trafficking.

He set aside a strict new ban on assault weapons and a limit on gun clip sizes, which had been aggressively promoted by Sen. Dianne Feinstein (D-Calif.) since December’s mass shootings at a Connecticut elementary school.

GOP leaders have previously not objected to beginning debate on the legislation, as the conservative trio is opposed to doing, and several Republicans continue to work with Democrats on coming up with a bipartisan compromise on background check legislation.

Reid’s spokesman said the Paul-Cruz-Lee letter was “not surprising” but missed the mark on the nation’s reaction to the Connecticut murders.

“It's outrageous that these senators are unwilling to even engage in a debate over gun violence in America. No matter your opinion on this issue, we should all be able to agree with President Obama when he said that the children and teachers of Newtown, along with all other Americans who have been victims of gun violence, at least deserve a vote,” said Adam Jentleson, Reid’s spokesman.

Under new rules approved at the start of the calendar year, Reid could conceivably move around the threatened filibuster by bypassing the motion to proceed and jump onto the legislation. In doing so, he would have to guarantee that Republicans get to offer at least two amendments to the bill. That new procedure has not been used yet, and doing so could thwart any bipartisan momentum toward getting a deal that a large number of senators supported.