Retiring Sen. Herb Kohl (D-WI) endorsed Tammy Baldwin’s candidacy for the U.S. Senate on Monday as she opened her Senate campaign headquarters in Milwaukee, according to WISN.com

Baldwin, who is currently representing Wisconsin’s 2nd district in the U.S. House, is running for the seat being vacated by Kohl.

“We are going to see to it that Tammy Baldwin is elected as our senator,” said Kohl. “I would have no one else as my successor. So you better get elected as senator.”

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The 76-year-old senator announced in May that he would not seek reelection to a fifth term in the Senate, saying that it was “time for someone else to serve.

If Baldwin won, she would be the first openly gay U.S. senator. She became the first openly gay candidate to win election to the U.S. Congress in 1998.

Chuck Wolfe, president and CEO of the Gay & Lesbian Victory Fund, said her candidacy would be a “remarkable milestone for LGBT Americans.”

But Baldwin has insisted that she does not want her senatorial campaign to revolve around her sexuality.

She is the only Democrat to have declared her candidacy for the seat.

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Baldwin will face Tommy Thompson, Mark Neumann, Jeff Fitzgerald or Frank Lasee depending on who wins the Republican primary.