Democratic Congressman Chris Pappas has made history after winning New Hampshire's US Senate race, becoming the first openly gay man to be elected to the US Senate.

Pappas defeated Republican former Senator John E. Sununu in Tuesday's November 3 midterm elections, securing the seat previously held by retiring Democratic Senator Jeanne Shaheen.

Addressing the crowds at his election watch party, Pappas said: "I want to thank the guy next to me, my amazing husband. I'm the luckiest guy because I'm married to a man that I love.

"And despite all the negative attacks and the lies, it's nice to know that love and truth prevailed tonight."

His win keeps the seat in Democratic hands after Senator Jeanne Shaheen's retirement following 18 years in office.

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

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."