On March 21, 1950, an Illinois congressman named Ralph Church suddenly slumped in his seat while testifying before a House committee. His colleagues rushed to administer aid, but he was pronounced dead of a heart attack at 66.

He was neither the first nor the last member of Congress to die in office. “You look back in history, nearly one in 10 members of Congress have,” said Jane L. Campbell, president of the U.S. Capitol Historical Society.

That history has some Democrats worried that deaths or illnesses could derail President Biden’s efforts to pass ambitious bills through Congress, which his party controls by the narrowest margins in decades.

“Our ability to make good on Biden’s agenda is pretty much dangling by a thread,” said Brian Fallon, a former aide to Senator Chuck Schumer, the Democratic majority leader. “I don’t think it’s uncouth to talk about it. I think it’s a reality that has to inform the urgency with which we approach those issues.”