Sen. Chris MurphyChristopher (Chris) Scott MurphyAvalanche of Democratic senators say Franken should resign Senate passes tax overhaul, securing major GOP victory Dem senator compares GOP tax bill to unicorns, Tupac conspiracy theories 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) ManchinTrump rips Dems a day ahead of key White House meeting Senate panel moves forward with bill to roll back Dodd-Frank Wealthy outsiders threaten to shake up GOP Senate primaries MORE (W.Va.), Patrick LeahyPatrick Joseph LeahyAvalanche of Democratic senators say Franken should resign America isn't ready to let Sessions off his leash Your tax dollars fund Afghan child rape MORE (Vt.), Al FrankenAlan (Al) Stuart FrankenDemocrats turn on Al Franken Schumer called, met with Franken and told him to resign Overnight Finance: Trump says shutdown 'could happen' | Ryan, conservatives inch closer to spending deal | Senate approves motion to go to tax conference | Ryan promises 'entitlement reform' in 2018 MORE (Minn.), Cory Booker (N.J.), Bill NelsonClarence (Bill) William NelsonOvernight Health Care: Ryan's office warns he wasn't part of ObamaCare deal | House conservatives push for mandate repeal in final tax bill | Dem wants probe into CVS-Aetna merger Ryan's office warning he wasn't part of deal on ObamaCare: source Overnight Health Care: Funding bill could provide help for children's health program | Questions for CVS-Aetna deal | Collins doubles funding ask for ObamaCare bill MORE (Fla.), Dick DurbinRichard (Dick) Joseph DurbinDemocrats turn on Al Franken Minnesota's largest newspaper calls on Franken to resign Democratic senator predicts Franken will resign Thursday MORE (Ill.), Charles SchumerCharles (Chuck) Ellis SchumerAmerica isn't ready to let Sessions off his leash Schumer celebrates New York Giants firing head coach: ‘About time’ GOP should reject the left's pessimism and the deficit trigger MORE (N.Y.), Ben CardinBenjamin (Ben) Louis CardinDems demand Tillerson end State hiring freeze, consult with Congress Former New Mexico gov: Trump's foreign policy is getting 'criticized by everybody' Dems put hold on McFarland nomination over contradictory testimony: report MORE (Md.) and Ed MarkeyEdward (Ed) John MarkeyNet neutrality supporters predict tough court battle over FCC's repeal plan Avalanche of Democratic senators say Franken should resign Driverless car bill hits Senate speed bump MORE (Mass.).

It also won support from presumptive Democatic presidential nominee Hillary ClintonHillary Diane Rodham ClintonGrassley blasts Democrats over unwillingness to probe Clinton GOP lawmakers cite new allegations of political bias in FBI Top intel Dem: Trump Jr. refused to answer questions about Trump Tower discussions with father MORE and her rival, Sen. Bernie SandersBernard (Bernie) SandersSchumer: Franken should resign Franken resignation could upend Minnesota races Avalanche of Democratic senators say Franken should resign 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 McConnellGOP strategist donates to Alabama Democrat McConnell names Senate GOP tax conferees Brent Budowsky: A plea to Alabama voters 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 blasts Democrats over unwillingness to probe Clinton Avalanche of Democratic senators say Franken should resign Blumenthal: ‘Credible case' of obstruction of justice can be made against Trump 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 CornynMcConnell names Senate GOP tax conferees Air Force makes criminal reporting changes after Texas massacre We need a better pathway for allowing civilians to move guns across state lines 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.