Sarah Silbiger/The New York Times

WASHINGTON — House Democrats are poised this week to adopt legislation requiring background checks for all gun purchasers, the first major expansion of gun control laws in a quarter century and the opening salvo in a broader drive to address an issue that once sharply divided the party.

In back-to-back votes, the House will take up two separate background check measures. On Wednesday it will consider the Bipartisan Background Checks Act, which would require background checks for all firearm sales, including those sold at gun shows and online. On Thursday it will turn to the Enhanced Background Checks Act, which would extend the time allotted for the F.B.I. to conduct background checks.

The action in the House reflects dramatic changes in the political climate over guns that culminated with last year’s midterm elections. A string of mass shootings, coupled with student-led activism after last year’s massacre at Marjory Stoneman Douglas High School in Parkland, Fla., produced a wave of Democratic candidates — including some from Republican-leaning districts — who ran and won while calling for limits on access to guns.

“It’s a real moment for the nation,” said one of those Democrats, Representative Lucy McBath of Georgia, who began advocating for gun control after her 17-year-old son, Jordan Davis, was shot dead in 2012. “We’re at a critical mass now where families are just saying they’ve had enough.”

Both measures face dim prospects in the Republican-controlled Senate. Senator John Thune of South Dakota, the No. 3 Republican, said Tuesday it was “unlikely” the Senate would take them up.

But this week’s House action is still significant. The last time Congress passed major gun control legislation was 1994, when an assault weapons ban was enacted, along with the so-called Brady Bill, which required background checks on most gun sales.

Democrats were punished roundly at the polls that year, with Republicans taking control of Congress and not relinquishing it for 12 years. For years afterward, Democrats were skittish about taking up gun-related bills.

The assault weapons ban expired in 2004, when Congress was under Republican control. When Democrats held the House from 2007 to 2011, no gun legislation was passed.

“There are still differences within our party as to how far we want to go,” Representative Steny D. Hoyer of Maryland, the House Democratic leader, told reporters on Tuesday. But Mr. Hoyer said he is convinced that the two bills being voted on this week have the support of the “overwhelming majority of the American people.”

Robin Lloyd, the managing director of the Giffords Law Center to Prevent Gun Violence, praised House Democrats for prioritizing the measures, noting that just a year ago, the Republican-controlled House tried — but failed — to pass legislation that would have required states to recognize concealed weapons permits issued in other states, effectively creating a national law allowing concealed weapons. That bill is a high priority for the National Rifle Association.

“Now we’re having the opposite conversation, and I do think that demonstrates how quickly this shift has happened,”…