Washington DC - Democratic Representative Chris Pappas won election on Tuesday to the Senate and will become the first openly gay man to serve in the upper chamber of Congress.

He defeated Republican former senator John Sununu to maintain Democratic control of the New Hampshire seat, US media projected.

The 46-year-old Pappas has served in the House of Representatives since 2018 and is co-chair of the Congressional Equality Caucus.

"No one should be discriminated against because of who they are or who they love," he said on his website, adding: "I am committed to fighting for the dignity and equality of all Americans."

US politics Midterms: Voters lean toward stinging rebuke of Trump as Democrats flip major Senate seat

The Human Rights Campaign, a major LGBTQ rights organization that endorsed Pappas, hailed his "historic win" and described him as a "dedicated pro-equality champion."

"We're ready to keep fighting for the rights of every Granite Stater alongside him," HRC said, using New Hampshire's nickname.

LGBTQ advocacy organization GLAAD also congratulated Pappas on his victory, saying he is "making history."

The midterm elections will determine whether Republicans keep control of Congress and will shape the rest of US President Donald Trump's second term.

Trump held near-daily rallies in the month before the midterms, urging Republicans to "pretend I'm on the ballot" despite his own record-low approval ratings.

But polls show Democrats have a good chance of seizing the House and possibly the Senate amid voter anger over the state of the US economy and the war against Iran.

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