Former Massachusetts Sen. Scott Brown (R) will announce he's running against Sen. Jeanne ShaheenCynthia (Jeanne) Jeanne ShaheenBiden administration resists tougher Russia sanctions in Congress GOP holds on Biden nominees set back gains for women in top positions Sununu setback leaves GOP scrambling in New Hampshire MORE (D-N.H.) in New Hampshire on Thursday, The Hill has confirmed.

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Brown's planned entrance into the race gives the GOP another pickup opportunity in a swing state, further expanding the map for the party as it looks to take back the Senate.

He has long been expected to run. Brown has flirted with a bid for months, moving from Massachusetts last year. He has been on a month-long "listening tour" in which he's publicly discussed a challenge to Shaheen, and said last week that he would definitely run against her.

Shaheen has led in most public polls by a high-single-digit margin.

WMUR first reported the news of Brown's decision. 

—This post was updated at 9:08 a.m. 

Alexandra Jaffe contributed to this report.