Sen. Chris MurphyChristopher (Chris) Scott MurphyChris Murphy’s profile rises with gun tragedies Murphy faces criticism from GOP challenger over fundraising email Democrat: Republicans who believe in more gun control afraid of being 'politically punished' 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 ManchinJoseph (Joe) ManchinOvernight Energy: EPA aims to work more closely with industry Overnight Finance: Lawmakers grill Equifax chief over hack | Wells Fargo CEO defends bank's progress | Trump jokes Puerto Rico threw budget 'out of whack' | Mortgage tax fight tests industry clout Lawmakers try again on miners’ pension bill MORE (W.Va.), Patrick LeahyPatrick Joseph LeahyOvernight Regulation: Massachusetts AG sues Equifax | Trump weighs easing rules on gun exports | EPA nominee to fight worker safety rule in court Trump to ease rules on gun exports: report Overnight Defense: Senate passes 0B defense bill | 3,000 US troops heading to Afghanistan | Two more Navy officials fired over ship collisions MORE (Vt.), Al FrankenAlan (Al) Stuart FrankenThe Hill's 12:30 Report The Hill's 12:30 Report John Oliver rips AT&T-Time Warner merger MORE (Minn.), Cory Booker (N.J.), Bill NelsonClarence (Bill) William NelsonSenate panel approves bill to speed up driverless cars Dems plan to make gun control an issue in Nevada Overnight Cybersecurity: Trump proclaims 'Cybersecurity Awareness Month' | Equifax missed chance to patch security flaw | Lawmakers await ex-CEO's testimony | SEC hack exposed personal data MORE (Fla.), Dick DurbinRichard (Dick) Joseph DurbinGun proposal picks up GOP support Durbin: I had 'nothing to do' with Curbelo snub Republicans jockey for position on immigration MORE (Ill.), Charles SchumerCharles (Chuck) Ellis SchumerOvernight Health Care: Schumer calls for tying ObamaCare fix to children's health insurance | Puerto Rico's water woes worsen | Dems plead for nursing home residents' right to sue Crying on TV doesn't qualify Kimmel to set nation's gun agenda Trump knocks ‘fake’ news coverage of his trip to Puerto Rico MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinOvernight Cybersecurity: Equifax CEO faces outraged lawmakers | Dem presses voting machine makers on cyber defense | Yahoo says 3 billion accounts affected by 2013 breach Key Dem: Did Kushner use private emails to talk with foreign governments? Dem senator pitches ideas for gun control after shooting MORE (Md.) and Edward MarkeyEdward (Ed) John MarkeyDems lambaste Trump’s ‘outrageous’ EPA chemical safety pick Dems plan to make gun control an issue in Nevada Dem senator pitches ideas for gun control after shooting MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonChris Murphy’s profile rises with gun tragedies DNC, RNC step up cyber protections Gun proposal picks up GOP support MORE and her rival, Sen. Bernie SandersBernard (Bernie) SandersChris Murphy’s profile rises with gun tragedies Clip shows Larry David and Bernie Sanders reacting after discovering they're related For now, Trump dossier creates more questions than answers 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 McConnellGun proposal picks up GOP support Children’s health-care bill faces new obstacles Dems see Trump as potential ally on gun reform 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 FeinsteinGun proposal picks up GOP support Gingrich: Banning rapid fire gun modification is ‘common sense’ House bill set to reignite debate on warrantless surveillance 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 CornynGun proposal picks up GOP support House bill set to reignite debate on warrantless surveillance Republicans jockey for position on immigration 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.