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.”