Sen. Mike EnziMichael (Mike) Bradley EnziGOP senators ask Trump for meeting on biofuels mandate Senate budget just the latest attack on seniors Week ahead: GOP's next steps on tax reform | Fed chief speculation heats up | Senate to vote on disaster relief MORE (R-Wyo.) on Monday joined a group of Republicans threatening to filibuster gun control legislation in the Senate.

“The measures proposed currently by the majority do not reduce crime, they simply restrict the American public’s constitutional right to self-defense,” Enzi said in a statement. “These bills would take away one of the basic freedoms guaranteed by the Bill of Rights and proponents won’t stop with these restrictions.”

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In March, a group of Republican lawmakers, led by Sens. Rand PaulRandal (Rand) Howard PaulCongress must end American support for Saudi war in Yemen Black men get longer prison sentences than white men for same crimes: study Sarah Palin on sexual harassment: 'People know I'm probably packing' so they 'don't mess with me' MORE (Ky.), Mike LeeMichael (Mike) Shumway LeeProminent conservative passes on Utah Senate bid Johnson says he will not support tax-reform bill Moore endorsements disappear from campaign website MORE (Utah), Ted CruzRafael (Ted) Edward CruzTexas Republicans slam White House over disaster relief request Dem rep: Trump disaster aid request is 'how you let America down again' Moore endorsements disappear from campaign website MORE (Texas) and Marco RubioMarco Antonio RubioCongress faces growing health care crisis in Puerto Rico The Hill's 12:30 Report Colbert mocks Trump for sipping water during speech on Asia trip MORE (Fla.), wrote to Senate Majority Leader Harry ReidHarry ReidVirginia was a wave election, but without real change, the tide will turn again Top Lobbyists 2017: Grass roots Boehner confronted Reid after criticism from Senate floor MORE (D-Nev.) pledging to “oppose the motion to proceed to any legislation that will serve as a vehicle for any additional gun restrictions.”

The group now counts 13 Republican senators vowing to filibuster gun control legislation.

“We, the undersigned, intend to oppose any legislation that would 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 group’s letter to Reid read.

Enzi’s support for the threatened filibuster comes ahead of a crucial test for gun control in the Senate. With lawmakers returning from a two-week Easter recess, Reid is poised to push a gun control bill on the Senate floor. But with Republican opposition intensifying, the bill faces a tough fight in the upper chamber.

Reid lashed out at the Republican senators threatening a filibuster in a speech Monday on the floor of the Senate.

“We should not stifle debate, run from tough issues or avoid difficult choices,” Reid said. “This body – the world’s greatest deliberative body – has a proud tradition of such robust and constructive debate.

The Democratic leader said he was “deeply troubled that a number of my Republican colleagues plan not only to oppose stricter gun violence laws, but to prevent the Senate from even voting on those measures.”

Reid accused the Republican signers of being “afraid to even engage in this debate,” and said they owed the families of the Sandy Hook victims a vote.

“Shame on them,” he said. “The least Republicans owe the parents of 20 children murdered with guns at Sandy Hook Elementary is a thoughtful debate about whether stronger gun laws could have saved their little girls and boys. The least Republicans owe them is a vote.”

The gun control bill would expand background checks and penalties on straw purchases of firearms. The portion of the bill on illegal trafficking of firearms has bipartisan support, including from Sen. Chuck GrassleyCharles (Chuck) Ernest GrassleyFBI informant gathered years of evidence on Russian push for US nuclear fuel deals, including Uranium One, memos show Klobuchar taking over Franken's sexual assault bill Lawyer: Kushner is 'the hero' in campaign emails regarding Russia MORE (Iowa), the ranking Republican on the Senate Judiciary Committee.

But the language on background checks, once called the “sweet spot” for reform by Sen. Charles SchumerCharles (Chuck) Ellis SchumerJuan Williams: The politics of impeachment Texas Republicans slam White House over disaster relief request Dem rep: Trump disaster aid request is 'how you let America down again' MORE (D-N.Y.), has seen increased opposition after his efforts to craft a bipartisan proposal fell short.

Republicans expressed fears the language could lead to the creation of a federal database of gun owners and apply to gun transfers between family members.

The Washington Post reported on Monday that Democratic Sen. Joe ManchinJoseph (Joe) ManchinDemocrats scramble to contain Franken fallout  Overnight Finance: House passes sweeping tax bill in huge victory for GOP | Senate confirms banking regulator | Mulvaney eyed for interim head of consumer agency Overnight Regulation: Senators unveil bipartisan gun background check bill | FCC rolls back media regs | Family leave credit added to tax bill | Senate confirms banking watchdog MORE (W.Va.) and his GOP colleague Sen. Pat Toomey (Pa.) are working on language to expand checks on firearm sales at gun shows and over the Internet while excluding family members and some hunters, a compromise they hope will be a way around the current deadlock.

Grassley is also preparing gun control legislation he will offer as an alternative to the package from Democrats, but he has yet to release new details.

The White House has been steadily ramping up its efforts in pressuring Congress to act, with President Obama taking his case on the road and hosting events with families of victims of gun violence.

Obama will speak in Connecticut on the importance of gun control Monday and will meet privately with the relatives of children killed in December in the mass shooting at Sandy Hook Elementary School. After the speech, the families will join Obama on Air Force One in a trip back to Washington.

On Tuesday, Vice President Biden and Attorney General Eric HolderEric H. HolderSessions defends Lynch's use of an email pseudonym: 'I have a pseudonym' Holder: Sessions is ‘racially insensitive’ and ‘racially unaware’ Let's start giving media manipulation the attention it deserves MORE will be joined by law enforcement officials in a joint speech about gun control at the White House.

Obama press secretary Jay Carney last week warned against a Senate filibuster, saying that such a move would be “unfortunate.”

I don't think you need to tell the families of those who have lost their children to gun violence that bills like this may be filibustered. I don't think that would be welcome news,” he told reporters.

Sen. John McCainJohn Sidney McCainTrump's dangerous Guantánamo fixation will fuel fire for terrorists Tech beefs up lobbying amid Russia scrutiny Ad encourages GOP senator to vote 'no' on tax bill MORE (R-Ariz.) on Sunday also criticized the GOP threats to filibuster gun legislation and called for a debate on the bill.

“I don’t understand it,” said McCain on CBS’s “Face the Nation.” “The purpose of the United States Senate is to debate and to vote and to let the people know where we stand.”

This story was last updated at 4:13 p.m.