Demonstrators assemble outside the office of Senate Majority Leader Mitch McConnell (R-Ky.) in Louisville. (Luke Sharrett/Getty Images)

More than 200 House Democrats sent a letter to Senate Majority Leader Mitch McConnell (R-Ky.) on Wednesday urging him to bring the Senate back from August recess to pass a bill expanding background checks for gun purchases.

The letter follows similar demands from Senate Democrats who, in the immediate aftermath of two mass shootings in Texas and Ohio, called on McConnell to take up a House version of the legislation, which passed the lower chamber in February.

“Any delay to pass commonsense gun violence prevention legislation only increases the chances that more innocent people in America may suffer from the tragic and needless loss caused by gun violence,” wrote the Democratic House members.

It has been 150 days since the House passed its legislation, the Democrats note, and in that time “countless lives have been lost.”

Separately, House Speaker Nancy Pelosi (D-Calif.) addressed the issue in a “Dear Colleague” letter to the House Democrats reviewing the legislative priorities of the caucus.

“As our nation mourns the loss of life from the gun violence of the past week, House Democrats are moving prayerfully and purposefully to advance action to address the national and personal security threat of white supremacy, domestic terrorism and gun violence in America,” Pelosi wrote.

Pelosi also noted that House Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) will be looking at advancing other measures to prevent gun violence, including one on “red-flag laws” that allows authorities to confiscate firearms from someone deemed to be a risk to themselves or the public.

McConnell has shown no willingness to bring up the House background-checks bill, let alone call the Senate back to do so. Even if he did, there are not enough votes in the GOP-controlled Senate to pass the House version.

The last time Congress tried to pass a background-checks bill was after the December 2012 Sandy Hook Elementary School shooting in Newtown, Conn. At that time, Sens. Patrick J. Toomey (R-Pa.) and Joe Manchin III (D-W.Va.) partnered on a bill that was defeated by six votes.

That bill included some concessions to gun rights advocates that were not included in the most recent House version, making it even more challenging to win Republican support.