Sen. Chris MurphyChristopher (Chris) Scott MurphyGraham's COVID-19 'breakthrough' case jolts Senate Democrats warn shrinking Biden's spending plan could backfire Democrats ramp up pressure for infrastructure deal amid time crunch MORE (D-Conn.) and other Democrats have taken over the Senate floor to call for tougher gun control laws and specifically action on keeping people on terrorist watchlists from buying firearms.

“I'm prepared to stand on this floor and talk about the need for this body to come together on keeping terrorists away from getting guns ... for, frankly, as long as I can, because I know that we can come together on this issue,” Murphy said in beginning the filibuster on Wednesday.

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Murphy began speaking at about 11:20 a.m., and the filibuster was still going more than 12 hours later.

Other Democrats who joined him included Sens. Richard Blumenthal (Conn.), Joe ManchinJoe ManchinSenate rejects GOP effort to add Trump border wall to bipartisan infrastructure deal Youth organizations call on Biden to ensure 'bold' climate investments Democrats barrel toward August voting rights deadline MORE (W.Va.), Patrick LeahyPatrick Joseph LeahySenate panel advances first three spending bills McConnell lays out GOP demands for government-funding deal The Hill's Morning Report - Presented by Facebook - Biden sets new vaccine mandate as COVID-19 cases surge MORE (Vt.), Al FrankenAlan (Al) Stuart FrankenCould Andrew Cuomo — despite scandals — be re-elected because of Trump? Al Franken to launch 15-stop comedy tour Democrats, GOP face crowded primaries as party leaders lose control MORE (Minn.), Cory Booker (N.J.), Bill NelsonClarence (Bill) William NelsonTom Brady to Biden: '40 percent of the people still don't think we won' Rubio, Demings rake in cash as Florida Senate race heats up How transparency on UFOs can unite a deeply divided nation MORE (Fla.), Dick DurbinDick DurbinSenate eyeing possible weekend finish for T infrastructure bill The Hill's Morning Report - Presented by Facebook - Cuomo defiant as Biden, Democrats urge resignation Democrats barrel toward August voting rights deadline MORE (Ill.), Charles SchumerChuck SchumerYouth organizations call on Biden to ensure 'bold' climate investments New York Times calls on Cuomo to resign 'The Squad' celebrates Biden eviction moratorium MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinSenate panel votes to repeal Iraq war authorizations Senate Democrats press administration on human rights abuses in Philippines Democrats pushing for changes to bipartisan infrastructure deal MORE (Md.) and Ed MarkeyEd MarkeySenate Democrats to introduce measure taxing major polluters Democrat senators introduce resolution for COVID-19 memorial day Human rights can't be a sacrificial lamb for climate action MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonBriahna Joy Gray: Progressives like Turner should reconsider running as Democrats Biden wishes Obama a happy birthday Ohio special election: A good day for Democrats MORE and her rival, Sen. Bernie SandersBernie SandersBriahna Joy Gray: Progressives like Turner should reconsider running as Democrats Senate Democrats to introduce measure taxing major polluters Biden called Shontel Brown to congratulate her after Ohio primary win MORE (I-Vt.).

The Senate is currently considering an appropriations bill for the Commerce and Justice departments and science programs. Though no votes are currently scheduled, the senators are blocking any amendments to the bill.

"I don't think we should proceed with debate on amendments to this bill until we have figured out a way to come together," Murphy said, referring to the spending bill.

The Democrats are also technically blocking Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellOn The Money: Trump asks court to block release of tax returns to Congress | Private sector adds 330K jobs in July, well short of expectations Senate panel advances first three spending bills McConnell lays out GOP demands for government-funding deal MORE (R-Ky.) from ending debate on the legislation, though a spokesman shot down any speculation that the Republican leader would have tried to wrap up work on the spending bill Wednesday.

Democrats have given no indication how long they plan to hold the Senate floor. Chris Harris, a spokesman for Murphy, said Democrats launched the talkathon because the senator will no longer accept “inaction or half measures in the face of continued slaughter.”

The effort comes three days after the deadliest mass shooting in U.S. history. A gunman armed with a Sig Sauer MCX rifle and a handgun on early Sunday killed 49 people at an Orlando nightclub. 

Murphy is well-known for his support for tougher gun control laws. One of America's most shocking gun crimes occurred in his home state in Newtown, where 20 children and six adults were shot to death at Sandy Hook Elementary School.

"I can't tell you how hard it is to look into the eyes of the families of those little boys and girls who were killed in Sandy Hook and tell them that almost four years later we've done nothing, nothing at all," Murphy said. 

Since the Orlando attack, Democrats have put a renewed focused on legislation meant to block the sale of guns to people on terrorist watchlists. 

An effort to move legislation was previously blocked last year. 

Sen. Ben Sasse (Neb.) was the first Republican senator to join Murphy and other Democrats, asking a question about the government's terror watchlists.

"I'm familiar with the terrorist screening database. There are a series of lists that fall from the database, but I don't think there's any such thing as 'the terrorist watchlist,' and I certainly don't understand what due process rights would apply," Sasse said.

Sasse's comment reflects a central problem Republicans have with a proposal from Sen. Dianne FeinsteinDianne Emiel FeinsteinNearly 140 Democrats urge EPA to 'promptly' allow California to set its own vehicle pollution standards Biden signs bill to bolster crime victims fund Stripping opportunity from DC's children MORE (D-Calif.) that would give the attorney general broad authority to block people on watchlists from being able to buy guns or explosives. 

Republicans argue that could deny constitutional rights to Americans who aren’t actually tied to terrorism.

Instead, Sasse and most Republicans support an alternative proposal by Sen. John CornynJohn CornynWhite House trying to beat back bipartisan Cornyn infrastructure amendment Senate GOP shifts focus to fight over Biden's .5 trillion budget McConnell warns Schumer cutting off debate quickly could stall infrastructure deal MORE (R-Texas) that would allow the attorney general to delay suspected terrorists from getting a gun for up to 72 hours while seeking a court order to stop the sale.

Sen. Pat Toomey (R-Pa.), who faces a tough reelection bid, came to the Senate floor urging his colleagues to compromise. 
 
"There's an obvious opportunity here, guys, to work together and find the solution," he said. "What I'm suggesting is let's get to work here." 
 
Toomey on Wednesday criticized the Feinstein measure as "badly flawed." But he also said that Cornyn's proposal, which he previously voted for, likely didn't give enough leeway to the attorney general. 
 
He made the comments after Democratic Senate candidate Katie McGinty, who is hoping to unseat Toomey, pressured the GOP senator to back the Feinstein legislation during a press conference Tuesday. 
 
Wednesday afternoon, Toomey said he planned to introduce a compromise bill Thursday aimed at combining aspects of both the Feinstein and Cornyn proposals.
 
Toomey's legislation would require the attorney general to create a list of "likely terrorists" that could be blocked from buying guns. The list would then be submitted to the Foreign Intelligence Surveillance Court, which would review it annually and remove any Americans inadvertently included, according to a summary of the forthcoming bill from his office.

Updated at 11:35 p.m.