The U.S. Capitol dome is seen on March 21 in Washington. (Jabin Botsford/The Washington Post)

Mississippi Republican Cindy Hyde-Smith was sworn in to the Senate on Monday, becoming the first woman to represent her state in the upper chamber of Congress.

Vice President Pence swore in Hyde-Smith in the Senate chamber Monday afternoon. She replaces Republican Thad Cochran, a longtime lawmaker who stepped down earlier this month amid health problems.

She is also running in a November special election to fill out the remainder of Cochran’s term, which expires in January 2021.

Now that Hyde-Smith has joined the Senate, the number of women in the chamber has grown to 23, a new high.

The race has become a key front in the battle for the Senate majority. Republicans are defending a narrow 51-to-49 majority. A quickly growing field in the state has created unpredictability and kept alive the prospect of a Democratic upset.

There will be no partisan primaries ahead of the Nov. 6 election. Instead, all candidates will appear on the ballot. If no one receives a majority, the top two finishers — regardless of party — will advance to a runoff.

Aside from Hyde-Smith, the candidates include Chris McDaniel, an insurgent conservative Republican who ran against Cochran in 2014 and has been hostile to Senate GOP leaders; Democrat Mike Espy, who was Mississippi’s first black member of Congress since Reconstruction; and Tupelo, Miss., Mayor Jason Shelton, another Democrat.

Senate Majority Leader Mitch McConnell (R-Ky.) and President Trump urged Gov. Phil Bryant (R) to consider appointing himself and running for the seat. But he refused and opted in March to appoint Hyde-Smith. Neither McConnell nor Trump has endorsed Hyde-Smith in the special election.

Before joining the Senate, Hyde-Smith served as state agriculture and commerce commissioner. She is also a former Democrat. Her past party affiliation has drawn criticism from McDaniel. Some mainstream Republicans fear it could be an effective line of attack in Mississippi, which is a heavily conservative state.