Four-term Congressman Chris Pappas of New Hampshire has announced his bid to succeed retiring U.S. Sen. Jeanne Shaheen, a fellow Democrat, next year.

Mr. Pappas, a 44-year-old Democratic congressman, is the first major candidate to enter the race to succeed the retiring Senator Jeanne Shaheen.

Rep. Chris Pappas is entering the race for New Hampshire’s open Senate seat. The fourth-term congressman is the first high-profile candidate to launch a campaign to succeed Sen. Jeanne Shaheen, a fellow Democrat who is retiring after three terms.