WASHINGTON — One of Congress’s most well-documented and visible responsibilities is to simply vote. On each bill, dozens of votes may be required and they are usually recorded.

But sometimes, members don’t show up for them. Sometimes they are back in their district, or away for personal reasons. When they don’t show, the “missed vote record” adds up.

Missed votes are often used as measures of “not doing their job” by rival candidates when House members run for reelection, said Jennifer Duffy, senior editor at the Cook Political Report, a nonpartisan organization that tracks campaigns.

“Voters fundamentally expect people to show up for work,” Duffy said. “That’s how they measure Congress’s work.”

But campaigns use the number delicately because it can backfire, Duffy said. “Some campaigns use it and fail because the number isn’t very impressive or very often the case they accuse someone of missing a vote, and they didn’t bother to research why.”

The lifetime median for the percentage of missed votes in Congress hovers at 2.3 percent, according to GovTrack.us.