Several were Never Trumpers, as Ms. Matthews describes herself, and about half had been to multiple postcard parties to support Democratic women in other congressional districts. Some had begun writing postcards in Virginia’s 2017 elections, when female candidates had unusual success challenging incumbents in the state’s House of Delegates.

“Everyone’s mood was lifted because we’re all doing something that’s very tangible,” Ms. Matthews said. “You’ve got the names of these voters; it’s not the hopeless ‘I’m writing to my congressman’ thing.”

(Activism has run high in Ms. Spanberger’s race. “The women are in my grill no matter where I go,” her opponent, Representative Dave Brat, complained to fellow Republicans in January 2017.)

Alarmed by Mr. Trump’s threats to pull the United States out of the Paris climate agreement, Ms. Matthews and two friends — one a Democrat, one a Republican — started 1PlanetWomen to try to make women into climate voters.

They are working with Virginia garden clubs to talk about how climate change affects women’s immediate lives — they cannot enjoy their gardens, for example, with ticks and mosquitoes having longer seasons, and more children have asthma — and have begun running targeted Instagram ads supporting Ms. Spanberger. To start the group, they turned to their contact lists of women from school, the neighborhood and church.

“Some of this reflects that women tend to have more of an active social network to begin with,” said Leah Greenberg, the co-founder and co-executive director of Indivisible, an incubator for resistance groups across the country. Roughly 75 percent of local Indivisible leaders and members are women.