To her surprise, it passed.

‘You Have to Believe in Something’

Shortly after the vote, Pamela Merritt’s cellphone started pinging with texts. An abortion rights activist who was a member of the platform committee, Ms. Merritt had not attended the vote, but started hearing about it almost immediately from angry friends demanding an explanation.

“My stomach dropped,” said Ms. Merritt, who had agreed to join the committee after the party’s steep losses of 2016, thinking she needed to do more than criticize from the sidelines.

In her view, Missouri Democrats needed more progressive politics, not less.

“I don’t understand Democrats who quote Truman and F.D.R. and then act like they are terrified to run as an actual Democrat,” said Ms. Merritt, 45, who lives in St. Louis. “You have to believe in something in order for somebody to believe in you. You can’t be such a watered-down thing.”

The fight over abortion in the party, she said, epitomized that. So she sprang into action, talking on Facebook and Twitter with hundreds of angry progressives, some of whom were threatening to stop their donations, calling her fellow committee members, and ultimately the party’s chairman.

“I felt horrified that someone would associate me with that bizarre, regressive anti-woman language,” she said.

The party was trying to placate people who opposed abortion at the very moment that abortion was most under threat, Ms. Merritt said. Days before the vote, Supreme Court Justice Anthony Kennedy had announced his retirement and the court had backed anti-abortion pregnancy centers. Missouri, one of the most restrictive states in the country, is now down to one abortion clinic.

“The last thing we needed was for that language to linger,” she said of the plank. “It was a foul stench that needed to be addressed sooner rather than later.”