A record number of women have already dominated elections this year — from Virginia to Michigan to California — and they could be the key to Democratic success on Election Day.

In the past, women’s representation across different states has been far from even, however.

Several states including Mississippi have only sent a woman to Congress in the last decade. Vermont still has yet to do so. Meanwhile, Montana, California, and Georgia were among some of the first to elect or appoint a woman to the House. (Today, Georgia could become a history-making state once more, with Stacey Abrams running to be the first black woman governor in America.)

Kelly Dittmar, a scholar at Rutgers’s Center for American Women and Politics, says a combination of factors could make some states better at promoting women than others.

Frontier states led the way on women’s suffrage and were among the first to send women to Congress

Some of the earliest states to send women to Capitol Hill were both “frontier states” and among the first to approve women’s suffrage.

These places were likely more open to promoting women in roles outside domestic spaces, Dittmar says.

“We see some of the earliest women broadly out west, in part, because of the frontier culture and the reality that there weren’t as many men,” she notes. “There was a little more of people being accustomed to seeing women outside traditional domestic spheres.”

This description certainly rings true for Montana, which was the first to send a woman to Congress in 1916, when voters elected suffragist Jeannette Rankin to the House. Rankin joined Congress three years before women across the country obtained the right to vote and was a key proponent for a constitutional amendment establishing women’s suffrage while she served in the House.

After the 19th Amendment was ultimately ratified in the early 1920s, California, Georgia, Illinois, and Oklahoma quickly sent women to Congress as well.

There have been surges in women joining Congress

Over time, the number of women in Congress has ebbed and flowed, with spikes in the 1950s, the 1970s, and a major surge in the early 1990s.

Dittmar notes that the women’s liberation movement likely contributed to the spike in the ’70s, while the allegations of sexual harassment that Anita Hill raised against Clarence Thomas in 1991 spurred more women to pursue office and usher in a “Year of the Woman” in 1992.

The optics of the hearings when Hill testified in front of an all-white, all-male Senate Judiciary panel were particularly stark, prompting many women including the eventual Sens. Patty Murray (D-WA) and Dianne Feinstein (D-CA) to run, historian Michele Swers said.

“A spotlight was shed on the underrepresentation of women in Congress where women are literally seeing the underrepresentation every day on the Senate Judiciary Committee,” said Dittmar. “That creates an impetus for women to say that we need more women in these spaces.”

Women make up just 20 percent of Congress

Iowa and Delaware were some of the most recent states to add women to their congressional delegations.

Some states may have lagged behind others in part because they have smaller delegations and fewer opportunities for women to break into these roles, Dittmar said. Vermont, the only state that has yet to elect or appoint a woman to Congress, has just three members in its delegation, for example. All three members have also been serving in those roles for at least a decade.

Larger state legislatures, like New Hampshire’s, can often offer a more accessible springboard for women seeking higher offices, though they’re not always tied with how many women a state elects to Congress.

Today, women’s representation in Congress still clocks in at just 20 percent of the overall body, but it has changed and grown significantly in recent decades.

As NPR’s Danielle Kurtzleben reports, there had previously been a longstanding trend of women obtaining elected office by taking over congressional terms that had been vacated by their deceased husbands. Today, more and more women are pursuing elected office in their own right.

Women’s representation has also become increasingly more diverse, with it expected to become even more so after this year’s midterm elections. Thirty-five percent of the women currently serving in Congress are women of color, according to CAWP.

Since Rankin took office more than 100 years ago, 329 women have been elected to the House or Senate and roughly 64 percent of them have been Democrats. This number could see a major spike on Tuesday.

“I may be the first woman member of Congress,” Rankin said when she was elected in 1916. “But I won’t be the last.”