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(CNN) —  

A record number of Senate races this year feature female nominees for both of the major parties, in another sign of the political success women are having heading into the midterms.

There are a record-breaking six races that currently feature women as both the Democratic and Republican nominees, doubling the previous record of three during the 2012 campaign.

One more could be on the way – the Massachusetts primary on Tuesday could add Sen. Elizabeth Warren, uncontested for the Democratic nomination, against Republican Beth Lindstrom, who’s vying with two men for her party’s nomination (Lindstrom was endorsed by The Boston Globe).

Of the states where women are running against each other – Arizona, Minnesota, Nebraska, New York, Washington and Wisconsin – four of the races feature Democratic incumbents (MN, WI, NY, WA), one features a Republican incumbent (NE) and one is an open race (AZ). Minnesota Sen. Tina Smith is running in a special election against Republican Karin Housley in the seat she was appointed to fill earlier this year.

All 11 incumbent female senators whose seats are on the ballot this year are running for re-election.

There have been only 52 female senators in US history; the most ever are the 23 presently serving in the upper chamber. These 23 women (17 Democrats and six Republicans) represent 19 states. California, Minnesota, New Hampshire and Washington are the only states where both sitting senators are women.

Before 2018, there had been only 16 Senate races total that featured two women as the major parties’ nominees. Democrats won 10 of them, and Republicans won six.

The first Senate election that featured two women as the major party nominees was in Maine in 1960. Republican Sen. Margaret Chase Smith was challenged by Democrat Lucia Cormier in her bid for a third term. According to the Senate’s historic webpage, Maine’s junior senator, Democrat Edmund Muskie, thought the only way to challenge Smith was to run another woman against her. He led the charge to nominate Cormier.

Cormier was a state representative and Democratic floor leader in the Pine Tree State. Time magazine did a cover story on the historic race. Smith won handily with 62% of the vote – the highest percentage of any Senate Republican that year.

It took another 26 years for two women to run against each other in a Senate race, when then-Rep. Barbara Mikulski faced former Reagan White House official Linda Chavez for the open Maryland seat in 1986. Mikulski won the first of five terms that year, beginning her 30-year career in the upper chamber, which would eventually make her the longest-serving female senator in history. She retired last year.

CNN’s Annie Grayer contributed to this report.