U.S. lawmakers returned to Congress on Monday after a summer rocked by mass shootings, with Democrats demanding President Donald Trump and his Republicans take action to restrict access to guns.

Senate and House members face a packed near-term legislative agenda, including the need to fund the government by October 1 or risk a shutdown.

The issue of impeachment, the tool a growing number of Democrats support using as a means to try to oust Trump from office, is also simmering, as House Democrats announced steps that could soon formalize the impeachment process.

But guns top the Democrats' immediate to-do list. Party leaders say they aim to mark up three gun control bills this week, and are pressing for the Senate's Republican leadership to allow a vote on a landmark background check bill that cleared the House earlier this year.

Both parties are acknowledging that any significant action depends on Trump.

Senate Majority Leader Mitch McConnell has said he will only bring a gun bill to the floor if it has presidential backing, but Trump has given no clear preference.

Top Senate Democrat Chuck Schumer convened a press conference Monday where he, House Speaker Nancy Pelosi and others demanded a vote on expanding background checks, which Schumer said would save many lives but which has languished in the Senate.

"Two people in Washington can make sure the background check bill passes: Donald Trump and Mitch McConnell," Schumer said.

"It is totally up to them," he added. "They can't escape that responsibility."