Added the entrepreneur Andrew Yang: “As the parent of two school age boys, this is our worst nightmare come to life.”

Gabrielle Giffords and Katie Hill call for stricter gun laws.

Gabrielle Giffords, the former congresswoman from Arizona who was shot in the head by a gunman in 2011 and has since become an advocate for stricter gun-control laws, called on President Trump and Republican lawmakers to take action after what she said was the 366th mass shooting so far in 2019.

“Americans have had enough of living in fear that today gun violence will change their lives,” Ms. Giffords said in a statement, adding that national leaders “can’t ignore the nightmare this public safety threat creates any longer.”

Ms. Giffords called on the Senate to pass a background checks bill approved by the House in February. The bill, which would require background checks for all gun purchasers, including those at gun shows and on the internet, was the first significant gun control bill to clear the chamber in a quarter-century.The legislation was propelled by last year’s massacre at Marjory Stoneman Douglas High School in Parkland, Fla., which prompted a wave of student-led activism that pressed Democrats to unite around gun control.

“Every politician paid to defend the status quo by the gun lobby needs to answer whether they are comfortable with live shooter drills becoming routine, students running terrified from their classrooms, and entire communities being locked down,” Ms. Giffords said, adding, “How many more deaths will happen before they sign that lifesaving legislation into law?”

Katie Hill, who graduated from Saugus High School in 2004 and represented the area in Congress before resigning earlier this month, said Santa Clarita has higher number of gun owners than average across the country. During her tenure, she told MSNBC, the House of Representatives passed four pieces of gun control legislation that have yet to be taken up by the Senate.

“This is something that my colleagues and I talked about all the time,” she said from her backyard, as helicopters searching for the shooter circled overhead “When you go back to your communities, people want to know what you’re going to do, and we’re really stuck.”

Several of her campaign interns and volunteers go to the school, she said on Twitter: “Praying for all.”

Arit John, Matt Stevens, Heather Tal Murphy, Dana Goldstein, Richard A. Oppel Jr., Amy Harmon and Patrick J. Lyons contributed reporting. Susan C. Beachy contributed research.