Sens. Rand Paul, R-Ky., Mike Lee, R-Utah, and Ted Cruz, R-Texas, are vowing to filibuster any new restrictions on guns, according to a letter the conservative trio wrote to Senate Majority Leader Harry Reid.
The warning comes just days after Reid announced plans to forge ahead on legislation that would expand background checks and stiffen penalties for gun trafficking after the Senate returns from a two-week Easter recess.
"We, the undersigned, intend to oppose any legislation that infringe on the American people's constitutional right to bear arms, or on their ability to exercise this right without being subjected to government surveillance," the senators wrote in the letter, a copy of which was obtained by Fox News.
Although the gun-control bill Reid outlined last week does not include a controversial assault weapons ban, Reid has promised to hold a vote on that proposal, as well as limits on high-capacity magazines, as an amendment to the bill.
Reid told reporters the assault weapons ban was kept out of the bill because the measure lacked the votes to pass.
Paul, Lee and Cruz do not address specific proposals in their letter, which is only three paragraphs long.
"The Second Amendment to the Constitution protects citizens' right to self-defense. It speaks to history's lesson that government cannot be in all places at all times, and history's warning about the oppression of a government that tries," they wrote, concluding, "We will oppose the motion to proceed to any legislation that will serve as a vehicle for any additional gun restrictions."
Always a thorny subject for Capitol Hill lawmakers, the debate over gun control has only grown more contentious in the wake of the December shooting at an elementary school in Newtown, Conn.
Over the weekend, New York City Mayor Michael Bloomberg announced that his advocacy group, Mayors Against Illegal Guns, is launching a $12 million ad campaign targeting senators in 13 key states.
That move drew a sharp rebuke from NRA chief Wayne LaPierre, who accused Bloomberg of trying to "buy America."