Sen. Chris MurphyChris MurphyGOP lawmakers defend Trump military rules of engagement Senate backs Montenegro's NATO membership Dem senator: GOP controls all of gov't, so success or failure is on them 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 BlumenthalRichard BlumenthalSenators introduce new Iran sanctions Senators demand Pentagon action after nude photo scandal Gorsuch rewrites playbook for confirmation hearings MORE (Conn.), Joe ManchinJoe ManchinSenate braces for fallout over Supreme Court fight Overnight Cybersecurity: First GOP lawmaker calls for Nunes to recuse himself | DHS misses cyber strategy deadline | Dems push for fix to cellphone security flaw Lawmakers call for pilot program to test for energy sector vulnerabilities MORE (W.Va.), Patrick LeahyPatrick LeahySenate braces for fallout over Supreme Court fight Register of copyrights should be presidential appointee GOP senator on going nuclear: 'I really hope that it doesn't come to that' MORE (Vt.), Al FrankenAl FrankenTax reform an important part of pro-consumer energy policy We need congressional debate on Yemen The case against Gorsuch: It’s all about precedent MORE (Minn.), Cory Booker (N.J.), Bill NelsonBill NelsonOvernight Finance: WH wants to slash billions | Border wall funding likely on hold | Wells Fargo to pay 0M over unauthorized accounts | Dems debate revamping consumer board Senators offer tax bill aimed at helping small businesses Path to 60 narrows for Trump pick MORE (Fla.), Dick DurbinDick DurbinRepublicans seek to lower odds of a shutdown No. 2 Senate Democrat opposes Trump's Supreme Court pick The Hill’s Whip List: 30 Dems are against Trump’s Supreme Court nominee MORE (Ill.), Charles SchumerCharles SchumerMcConnell: ObamaCare 'status quo' will stay in place moving forward NRA launches M Supreme Court ad Senate about to enter 'nuclear option' death spiral MORE (N.Y.), Ben CardinBen CardinSenate braces for fallout over Supreme Court fight Democrats step up calls that Russian hack was act of war Making water infrastructure a priority MORE (Md.) and Ed MarkeyEd MarkeySenate Dem: Trump is attacking science Overnight Energy: Trump signs climate order | Greens vow to fight back House passes bill undoing Obama internet privacy rule MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Rodham ClintonClinton targets Trump in speech, urging supporters to 'resist, insist, persist, enlist' Clinton defends April Ryan, Rep. Maxine Waters in speech Lobbying world MORE and her rival, Sen. Bernie SandersBernie SandersIn California race, social justice wing of Democrats finally comes of age Sanders to headline progressive 'People's Summit' The Hill's 12:30 Report 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 McConnellMitch McConnellScarborough: Bannon trying to ‘help his falling standing’ in WH Hatch: I may retire if Romney runs to replace me How the GOP’s ‘Access to Care’ bill cuts down states’ rights 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 FeinsteinOvernight Regulation: Trump repeals 'blacklisting' rule Dems delay Senate panel vote on Supreme Court nominee Dems get it wrong: 'Originalism' is mainstream, even for liberal judges 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 CornynSenate braces for fallout over Supreme Court fight Former congressman indicted on conspiracy charges No. 2 Senate Democrat opposes Trump's Supreme Court pick 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.