Sen. Chris MurphyChristopher (Chris) Scott MurphyHobbled NRA shows strength with Trump Overnight Defense: US, Russia tensions grow over nuclear arms | Highlights from Esper's Asia trip | Trump strikes neutral tone on Hong Kong protests | General orders ethics review of special forces White House eyes September action plan for gun proposals 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: Green groups sue Trump over Endangered Species Act changes | Bureau of Land Management retirees fight plan to relocate agency | Wildfires in Amazon rainforest burn at record rate Bureau of Land Management retirees fight plan to relocate agency out west GOP senator: Gun control debate 'hasn't changed much at all' back home MORE (W.Va.), Patrick LeahyPatrick Joseph LeahyAppropriators warn White House against clawing back foreign aid House panel investigating decision to resume federal executions Graham moves controversial asylum bill through panel; Democrats charge he's broken the rules MORE (Vt.), Al FrankenAlan (Al) Stuart FrankenNative American advocates question 2020 Democrats' commitment Reid says he wishes Franken would run for Senate again Al Franken urges Trump to give new speech after shootings: 'Try to make it sound like you're sincere, even if you're not' 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 DurbinSenate Democrats push Trump to permanently shutter migrant detention facility House panel investigating decision to resume federal executions To combat domestic terrorism, Congress must equip law enforcement to fight rise in white supremacist attacks MORE (Ill.), Charles SchumerCharles (Chuck) Ellis SchumerJewish Democratic congresswoman and veteran blasts Trump's 'disloyalty' comments Schumer says Trump encouraging anti-Semites Saagar Enjeti: Biden's latest blunder; Krystal Ball: Did Schumer blow our chance to beat McConnell? MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinAmerica is in desperate need of infrastructure investment: Senate highway bill a step in the right direction Financial aid fraud is wrong — but overcorrection could hurt more students Democrats denounce Trump's attack on Cummings: 'These are not the words of a patriot' MORE (Md.) and Ed MarkeyEdward (Ed) John MarkeyJoseph Kennedy mulling primary challenge to Markey in Massachusetts Overnight Energy: Trump sparks new fight over endangered species protections | States sue over repeal of Obama power plant rules | Interior changes rules for ethics watchdogs To cash in on innovation, remove market barriers for advanced energy technologies MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonPoll shows Biden, Warren tied with Trump in Arizona The Hill's Morning Report - Trump touts new immigration policy, backtracks on tax cuts Hickenlooper announces Senate bid MORE and her rival, Sen. Bernie SandersBernie Sanders2020 candidates have the chance to embrace smarter education policies Bernie Sanders Adviser talks criminal justice reform proposal, 'Medicare for All' plan Poll shows Biden, Warren tied with Trump in Arizona 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 McConnellMcConnell rejects Democrats' 'radical movement' to abolish filibuster Hickenlooper announces Senate bid Trump orders elimination of student loan debt for thousands of disabled veterans 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 FeinsteinTrump administration urges Congress to reauthorize NSA surveillance program The Hill's Morning Report - More talk on guns; many questions on Epstein's death Juan Williams: We need a backlash against Big Tech 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 CornynThe Hill's Morning Report - Trump hews to NRA on guns and eyes lower taxes The Hill's Morning Report - Trump on defense over economic jitters Democrats keen to take on Cornyn despite formidable challenges 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.