Sen. Chris MurphyChristopher (Chris) Scott MurphyOvernight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Overnight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Chris Murphy may oppose bipartisan health bill unless it addresses ObamaCare 'sabotage' 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) ManchinCritics say Interior's top lawyer came 'close to perjury' during Hill testimony Critics say Interior's top lawyer came 'close to perjury' during Hill testimony The Hill's 12:30 Report — Presented by MAPRx — Trump takes heat for remarks on help from foreign governments MORE (W.Va.), Patrick LeahyPatrick Joseph LeahyHouse Democrats close to finalizing border aid bill House Democrats close to finalizing border aid bill Senate panel approves .5B for Trump's border request MORE (Vt.), Al FrankenAlan (Al) Stuart FrankenThe Hill's 12:30 Report: Mexican officials scramble to avoid Trump tariffs The Hill's 12:30 Report: Mexican officials scramble to avoid Trump tariffs The Hill's Morning Report - Tariff battle looms as Trump jabs 'foolish' Senate GOP MORE (Minn.), Cory Booker (N.J.), Bill NelsonClarence (Bill) William NelsonPoll: Six Democrats lead Trump in Florida match-ups Poll: Six Democrats lead Trump in Florida match-ups How Jim Bridenstine recruited an old enemy to advise NASA MORE (Fla.), Dick DurbinRichard (Dick) Joseph DurbinOvernight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Overnight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Kushner meeting with senators to craft asylum deal MORE (Ill.), Charles SchumerCharles (Chuck) Ellis SchumerSenate confirms Trump judicial nominee criticized for being hostile to LGBT community Senate confirms Trump judicial nominee criticized for being hostile to LGBT community Democrats detail new strategy to pressure McConnell on election security bills MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinProposed bipartisan kidney legislation takes on kidney disease epidemic in America Lawmakers raise security concerns about China building NYC subway cars House votes to boost retirement savings MORE (Md.) and Ed MarkeyEdward (Ed) John MarkeyHillicon Valley: Senate sets hearing on Facebook's cryptocurrency plans | FTC reportedly investigating YouTube over children's privacy | GOP senator riles tech with bill targeting liability shield | FAA pushed to approve drone deliveries Hillicon Valley: Senate sets hearing on Facebook's cryptocurrency plans | FTC reportedly investigating YouTube over children's privacy | GOP senator riles tech with bill targeting liability shield | FAA pushed to approve drone deliveries Overnight Defense: House passes T spending package with defense funds | Senate set to vote on blocking Saudi arms sales | UN nominee defends climate change record MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonDemocrats bristle as Hicks appears for daylong Capitol Hill testimony Democrats bristle as Hicks appears for daylong Capitol Hill testimony Trump: 'So sad' Democrats are putting Hope Hicks 'through hell' MORE and her rival, Sen. Bernie SandersBernie SandersSanders denies tweet about corporate Democrats was dig at Warren Sanders denies tweet about corporate Democrats was dig at Warren Democrats asked to create ideal candidate to beat Trump pick white man: poll 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 McConnellOvernight Defense: House passes T spending package with defense funds | Senate set to vote on blocking Saudi arms sales | UN nominee defends climate change record Overnight Defense: House passes T spending package with defense funds | Senate set to vote on blocking Saudi arms sales | UN nominee defends climate change record Senate to vote Thursday to block Trump's Saudi arms 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 FeinsteinDemocrats detail new strategy to pressure McConnell on election security bills Democrats detail new strategy to pressure McConnell on election security bills Hillicon Valley: GOP senator wants one agency to run tech probes | Huawei expects to lose B in sales from US ban | Self-driving car bill faces tough road ahead | Elon Musk tweets that he 'deleted' his Twitter account 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 CornynSanders says Juneteenth should be a national holiday Sanders says Juneteenth should be a national holiday GOP frets about Trump's poll numbers 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.