Progressive senators on Sunday said they would welcome an opportunity to return to Washington, D.C. in the midst of Congress's August recess, in order to vote on gun control reform following two mass shootings in the course of a day.

Sens. Bernie Sanders (I-Vt.) and Sherrod Brown (D-Ohio) demanded that Senate Majority Leader Mitch McConnell call the Senate back into session to vote on H.R. 8 and H.R. 1112, the background check bills passed by the Democratic-led House in February. The bills would require background checks on all firearm sales across the country, and strengthen existing checks.

"The House of Representatives has passed background check legislation," Brown told Jake Tapper on CNN. "The Senate could meet tomorrow. I hope that Sen. McConnell would bring the Senate back tomorrow and pass the background check bill and send it to the president and the president must sign it. Period."

Thoughts and prayers are not enough. We must act. Mitch McConnell please call the Senate back to work tomorrow and let us vote on gun-safety laws. pic.twitter.com/S7uoyPbK06 — Sherrod Brown (@SenSherrodBrown) August 4, 2019

Police and federal investigators on Sunday were still examining how the accused shooters in El Paso, Texas and Dayton, Ohio obtained the military-style semi-automatic weapons they allegedly used to kill a total of 29 people, and whether background checks were involved.

But Sanders argued that universal background checks, which have the support of 90 percent of the public, would be "a first step to addressing our serious gun violence epidemic" and preventing another mass shooting.

Mitch McConnell should bring the Senate back into session immediately to pass HR 8, the gun safety bill that has already passed the House. That's a first step to addressing our serious gun violence epidemic. — Bernie Sanders (@BernieSanders) August 4, 2019

Sanders sent a petition to his supporters asking them to join in the call for McConnell to take action.

"My colleagues know that there is action we can take to stem this slaughter," the senator and 2002 presidential candidate wrote. "And we know this because ours is the only major country on Earth with this level of gun violence. We can do it this week. We can take steps that we know will make us safer. And we should."

Under current law, only licensed gun dealers are required to perform background checks. Even if a background check isn't finalized within three days, a gun sale is permitted to go through.

The three-day loophole allowed the shooter who killed nine people at a church in Charleston, South Carolina in 2015, to obtain his weapon. The Virginia Tech shooting in 2007 and an attack at a church in Sutherland Springs, Texas also could have been prevented with a stronger background check system, experts say.

McConnell offered "prayers" on Twitter after Saturday's shooting in El Paso. Shortly after he posted the message, the weekend's second shooting took place in Dayton.

In response, critics shared their disgust with the Senate leader.

"Keep tweeting platitudes," wrote journalist Brian Tyler Cohen "This is on you."

The House passed HR 8 and HR 1112, both for universal background checks, a common sense gun safety measure. You, and you alone, won’t even allow the Senate to CONSIDER voting on either bill. Keep tweeting platitudes. This is on you. — Brian Tyler Cohen (@briantylercohen) August 4, 2019

Their blood is on your hands pic.twitter.com/KPVOfa9QrH — Claude Taylor (@TrueFactsStated) August 4, 2019

Rep. Alexandria Ocasio-Cortez (D-N.Y.) and former Rep. Gabby Giffords—who was injured in a shooting in Arizona in 2011—joined Sanders and Brown in the call for McConnell to reconvene lawmakers to pass background check legislation on Monday.

I have no more words. I only have anger. President Donald Trump and Leader Mitch McConnell must call the Senate back from August recess immediately. We cannot afford to wait another day for lawmakers to address this horrific national public safety threat. #daytonshooting https://t.co/IX5WS39CFk — Gabrielle Giffords (@GabbyGiffords) August 4, 2019