Update 9:30pm: Senate Democrats have now taken their filibuster on gun rights on Wednesday to its 10th hour.

A little after 9pm, Maine Senator Angus King noted that 10 people have been killed by guns since the filibuster began this morning.

People on federal terrorist watch lists should not be able to buy guns.

That may sound like a pretty uncontroversial proposal. But it’s run aground in Congress, prompting Senate Democrats to join together for a filibuster on Wednesday in response to the massacre at the Pulse nightclub in Orlando.

I am prepared to stand on the Senate floor and talk about the need to prevent gun violence for as long as I can. I've had #Enough — Chris Murphy (@ChrisMurphyCT) June 15, 2016

Connecticut Sen. Chris Murphy began speaking around 11:20 am, and Democrats continued to hold the Senate floor close to 4:30 pm, according to multiple reports.

Here's a live stream of the Senate floor:

While on the floor, Murphy said that 244 people on the terrorist watch list tried buying a firearm in 2015, and 223 of them (around 90 percent) succeeded. (The suspected Orlando shooter, Omar Mateen, had been on the watch list but was later removed after the FBI finished its investigation, according to the LA Times.)

"It's impossible for Americans to understand," Murphy said.

Murphy has called for stricter gun control measures since 20 young children in his state were killed at the Sandy Hook Elementary School in 2012. Like Hillary Clinton and Barack Obama, Murphy is also calling for universal background checks for gun purchases.

Murphy continues to hold the floor, but other prominent Senate Democrats including Cory Booker, Elizabeth Warren, and Patrick Leahy have also taken turns asking questions on the floor.

Why Republicans have opposed closing the loophole

The proposal to prevent those on the terrorist watch list from buying guns has been DOA in the Republican-led Senate thus far. Per the Hartford Courant:

Murphy is seeking a vote on a bill from Sen. Dianne Feinstein, D-Calif., that would let the government bar sales of guns and explosives to people it suspects of being terrorists. Feinstein offered the amendment in December, a day after an extremist couple killed 14 people in San Bernardino, Calif., but the Republican-run Senate rejected the proposal on a near party-line vote.

Republicans have continued to oppose the idea in the wake of the Orlando shooting, saying they worry that doing so would infringe on the rights of gun holders. (For a full explainer on the proposal, read this explainer from Vox's Dara Lind.)

"We don't want terrorists to be able to walk into a gun store and buy a gun, but we don't want an innocent law-abiding citizen to be denied his Second Amendment rights because he's on a list with a bunch of terrorists," said Pennsylvania Sen. Pat Toomey, a Republican, in an interview with the Courant this week.

On Wednesday, the Atlantic noted that the American Civil Liberties Union has joined Republicans in opposing the initiative in part because those on terrorist watch lists haven't necessarily been accused or convicted of a crime.

Democrats may see an opening in attacking the "terrorist gun loophole"

But Democrats have argued those concerns are overblown and that the bill is particularly necessary to pass after Orlando, which was the deadliest mass shooting in US history.

Citing the shooting this fall at a college in his state, Oregon Sen. Ron Wyden said not closing the "terrorist gun loophole" would amount to not acting in the wake of the 9/11 terrorist attacks.

"People shouldn’t look at that as a partisan issue," he said. "Americans want to know why anybody would vote to allow individuals suspected of terrorist ties and motivations to purchase regulated firearms."

That call was widely echoed by several prominent Democrats on Wednesday:

We are 2 hours in on Sen floor. No business as usual. We want a vote on closing the terrorist gun loophole. #ENOUGH https://t.co/Jg7tlBO7My — Cory Booker (@CoryBooker) June 15, 2016

I’m standing up & speaking out to close the terrorist gun loophole. #TerrorGap. Listen @JoyCardinShow: https://t.co/uFbINuH5LV — Sen. Tammy Baldwin (@SenatorBaldwin) June 15, 2016

Despite their longstanding opposition to the bill, Democrats may be hoping that tying gun control to terrorism will shift the political calculus for Republicans.

It may be working. On Wednesday, Donald Trump said he’d be talking to the National Rifle Association about the question — bucking his otherwise hard-line stance on gun control:

I will be meeting with the NRA, who has endorsed me, about not allowing people on the terrorist watch list, or the no fly list, to buy guns. — Donald J. Trump (@realDonaldTrump) June 15, 2016

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