WASHINGTON — The top two Democrats in Congress called on Sunday for President Trump to defy the National Rifle Association and get behind legislation, already passed by the House but blocked in the Senate, to expand background checks to nearly all gun buyers.

With gun control high on Congress’s agenda as lawmakers return to Washington this week after their August recess, Speaker Nancy Pelosi and Senator Chuck Schumer, the Democratic leader, sent a joint letter to the president, telling him that his “urgent, personal intervention is needed to stem the endless massacres of our fellow Americans by gunfire” and that he had a “historic opportunity to save lives.”

The letter, which Ms. Pelosi and Mr. Schumer intend to follow with a news conference on Monday, is part of what Democrats say will be an all-out push for gun safety legislation this fall. In it, they asked Mr. Trump to express his support for H.R. 8, the bill passed by the House that requires background checks for private gun sales, including those at gun shows and over the internet. Senator Mitch McConnell, the Republican leader, has so far refused to take up the measure, which is strongly opposed by the N.R.A.

“We implore you to seize this moment when your leadership and influence over Republicans in Congress on the issue of guns is so critical,” they wrote in the letter, which was obtained by The New York Times. “Please do not squander it by acceding to N.R.A.-backed proposals or other weak ideas that will do nothing to stop the continuing, horrific spread of gun violence and may, in some cases, actually make our communities less safe.”