Sen. Chris MurphyChristopher (Chris) Scott MurphyKavanaugh impeachment push hits Capitol buzz saw Democrats seize Senate floor to protest gun inaction: 'Put up or shut up' Senate Democrats to hold the floor to protest inaction on gun violence 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) ManchinKavanaugh impeachment push hits Capitol buzz saw Democrats seize Senate floor to protest gun inaction: 'Put up or shut up' Senate Democrats to hold the floor to protest inaction on gun violence MORE (W.Va.), Patrick LeahyPatrick Joseph LeahyDemocrats press for action on election security The Hill's Morning Report — Biden steadies in third debate as top tier remains the same Overnight Defense: Dems grill Trump Army, Air Force picks | House chair subpoenas Trump Afghanistan negotiator | Trump officials release military aid to Ukraine MORE (Vt.), Al FrankenAlan (Al) Stuart FrankenThe Hill's Morning Report - What is Trump's next move on Iran? The Memo: Times correction gives GOP lifeline in latest Kavanaugh controversy Politicon announces lineup including Comey, Hannity, Priebus MORE (Minn.), Cory Booker (N.J.), Bill NelsonClarence (Bill) William NelsonAl Franken says he 'absolutely' regrets resigning Democrats target Florida Hispanics in 2020 Poll: Six Democrats lead Trump in Florida match-ups MORE (Fla.), Dick DurbinRichard (Dick) Joseph DurbinKavanaugh impeachment push hits Capitol buzz saw Democrats seize Senate floor to protest gun inaction: 'Put up or shut up' Pressley on Kavanaugh impeachment: 'Deeply disturbing' that a justice 'could have this many allegations' MORE (Ill.), Charles SchumerCharles (Chuck) Ellis SchumerSchumer, Pelosi push Trump to back universal background check bill Sinema says she would back Kennedy in race against Markey Democrats threaten to withhold defense votes over wall MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinCongress passes bill to begin scenic byways renaissance GOP lawmaker: 'Dangerous' abuse of Interpol by Russia, China, Venezuela Senators pressure Trump to help end humanitarian crisis in Kashmir MORE (Md.) and Ed MarkeyEdward (Ed) John MarkeyGreta Thunberg scolds Congress on climate action: 'I know you are trying but just not hard enough' Obama meets with Greta Thunberg: 'One of our planet's greatest advocates' Young insurgents aren't rushing to Kennedy's side in Markey fight MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonGOP struggles with retirement wave Overnight Energy: Trump to revoke California's tailpipe waiver | Democrats propose bill to revoke Trump endangered species rollback | Trump officials finalize rule allowing fewer inspectors at pork plants Mark Mellman: The most important moment in history? MORE and her rival, Sen. Bernie SandersBernie SandersJimmy Carter: 'I hope there's an age limit' on presidency 2020 candidates keep fitness on track while on the trail Mark Mellman: The most important moment in history? 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 McConnellDemocrats seize Senate floor to protest gun inaction: 'Put up or shut up' Democrats press for action on election security Hillicon Valley: Election security looms over funding talks | Antitrust enforcers in turf war | Facebook details new oversight board | Apple fights EU tax bill 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 FeinsteinGrassley: Kavanaugh classmate didn't contact Senate panel Trump court pick sparks frustration for refusing to answer questions This week: Congress returns for first time since mass shootings 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 CornynDemocrats press for action on election security On The Money: NY prosecutors subpoena eight years of Trump tax returns | Senators struggle to get spending bills off ground as shutdown looms | Progressive tax-the-rich push gains momentum | Trump faces dwindling leverage with China Senators struggle to get spending bills off ground as shutdown looms 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.