Sen. Chris MurphyChristopher (Chris) Scott MurphySenators in the dark on parliamentarian's decision Democratic senators call on Biden to support waiving vaccine patents Giffords group unveils gun violence memorial on National Mall 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 ManchinOn The Money: Treasury creates hub to fight climate change | Manchin throws support behind union-backed PRO Act | Consumer bureau rolls out rule to bolster CDC eviction ban Miners union to back Biden on green energy if it retains jobs Manchin throws support behind union-backed PRO Act MORE (W.Va.), Patrick LeahyPatrick Joseph LeahySenate Democrats call on Biden to restore oversight of semiautomatic and sniper rifle exports Bottom line Congress brings back corrupt, costly, and inequitably earmarks MORE (Vt.), Al FrankenAlan (Al) Stuart Franken#MeWho? The hypocritical silence of Kamala Harris The Hill's Morning Report - Presented by Facebook - Senate Dems face unity test; Tanden nomination falls Gillibrand: Cuomo allegations 'completely unacceptable' MORE (Minn.), Cory Booker (N.J.), Bill NelsonClarence (Bill) William NelsonHas the Biden administration abandoned the idea of a moon base? Cuba readies for life without Castro Why does Rep. Johnson oppose NASA's commercial human landing system? MORE (Fla.), Dick DurbinDick DurbinWhite House defends 'aspirational' goal of 62,500 refugees Biden on refugee cap: 'We couldn't do two things at once' For a win on climate, let's put our best player in the game MORE (Ill.), Charles SchumerChuck Schumer'Real Housewives of the GOP' — Wannabe reality show narcissists commandeer the party 'Building Back Better' requires a new approach to US science and technology Pew poll: 50 percent approve of Democrats in Congress MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinSenators in the dark on parliamentarian's decision When it comes to the Iran nuclear deal, what's a moderate Democrat to do? Battle lines drawn on Biden's infrastructure plan MORE (Md.) and Ed MarkeyEd MarkeyThe Hill's Morning Report - Presented by Facebook - All US adults can get vaccine; decision Friday on J&J vax Democratic Rep. Mondaire Jones calls on Breyer to retire Democratic senators call on Biden to support waiving vaccine patents MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonPelosi on power in DC: 'You have to seize it' Cuba readies for life without Castro Chelsea Clinton: Pics of Trump getting vaccinated would help him 'claim credit' MORE and her rival, Sen. Bernie SandersBernie SandersOvernight Energy: Treasury creates hub to fight climate change through finance | Sanders, Ocasio-Cortez introduce 'Green New Deal for Public Housing' | Don't attack Zoom for its Bernie Sanders federal tax bill Sanders, Ocasio-Cortez introduce 'Green New Deal for Public Housing' 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 McConnellHouse votes to condemn Chinese government over Hong Kong 15 Senate Republicans pledge to oppose lifting earmark ban It's not 'woketivism,' it's good business 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 FeinsteinSenate Democrats call on Biden to restore oversight of semiautomatic and sniper rifle exports Overnight Defense: Army moves to combat sexual crimes | Eight West Point cadets expelled | Democratic senators want to restrict F-35 sale to UAE A proposal to tackle congressional inside trading: Invest in the US 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 Cornyn15 Senate Republicans pledge to oppose lifting earmark ban Cornyn defends controversial tweet as not about Biden's competency Media complicity in rise of the 'zombie president' conspiracy 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.