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WASHINGTON - Senate Majority Leader Harry Reid, D-Nev., had some tough words for Republicans threatening to filibuster gun control legislation the full Senate will start working on this week: "Shame on them."

"Senate Republicans seem afraid to even engage in this debate," Reid said on the Senate floor of the Republicans threatening to filibuster the gun control legislation.

"The least Republicans owe the parents of these 20 little babies who are murdered at Sandy Hook is a thoughtful debate about who whether stronger laws could have saved their little girls and boys," Reid said. "The least Republicans owe them is a vote."

In late March, three Republican senators - Rand Paul of Kentucky, Ted Cruz of Texas and Mike Lee of Utah - sent Reid a letter threatening to filibuster the gun control legislation.

The letter this morning was re-delivered with a little more weight to it, with now 13 Republican signers, including Senators James Inhofe of Oklahoma, Richard Burr of North Carolina, Mike Enzi of Wyoming, Marco Rubio of Florida, Jerry Moran and Pat Roberts of Kansas, Ron Johnson of Wisconsin, Dan Coats of Indiana, and Mike Crapo and James Risch of Idaho.

In the letter to Reid, the senators write that they will "oppose the motion to proceed to any legislation that will serve as a vehicle for any additional gun restrictions." Republican aides expect more Republican support to the filibuster threat as the week progresses on Capitol Hill.

Reid said this morning that he found this letter "deeply troubling," and labeled it "obstruction."

"My Republican colleagues went so far as to send me a letter saying we will agree to nothing, there will be no debate, there will be nothing," Reid said mocking the filibuster threat. "We want you to do zero on anything dealing with stricter gun measures. They don't even want to let us vote on this. … There is simply no reason for this blatant obstruction except the fear of considering antiviolence proposals in full view."

Reid said every idea for gun control legislation should get a vote, a signal that he might have an open amendment process for the gun control legislation being considered on the Senate floor this week.

"Every idea should be debated and every issue should get a vote," Reid said. "From better mental health treatment, more secure schools, stronger background checks, ban on assault weapons, the size of magazines or clips and other issues should get a vote. There are strong feelings and deep disagreements about some of these measures, but every one of these measures deserves a vote, a yes or no. No hiding, no running from an issue that has captivated America."

In March, Reid dropped the controversial proposal to ban assault weapons from the base Democrat's gun control bill, though he promised the measure would get a vote as a standalone amendment.

The assault weapons ban was just one of the gun measures passed by the Senate Judiciary Committee. The committee also voted in favor of a bill to boost school safety money, a bill that would expand federal background checks, and a gun trafficking bill that makes "straw" purchasing - legally buying a gun to give to a criminal or someone not allowed to have one - illegal.