Former Massachusetts Sen. Scott Brown (R) will announce he's running against Sen. Jeanne ShaheenCynthia (Jeanne) Jeanne ShaheenDems demand Tillerson end State hiring freeze, consult with Congress The Hill Interview: GOP chairman says ‘red flags’ surround Russian cyber firm Schumer celebrates New York Giants firing head coach: ‘About time’ MORE (D-N.H.) in New Hampshire on Thursday, The Hill has confirmed.

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.