Smith told WMUR-TV, Manchester, Sunday said he was encouraged to run after no major candidates came forward for several weeks.
"I have a decision to run again for the Senate," Smith said. "This is nothing personally against Jeanne Shaheen, who I like as a person, but I couldn't disagree more with her politics."
He said he will file paperwork needed to form his campaign committee soon, move back to New Hampshire full time and formally launch his campaign in January. Smith now is a Florida resident, but spends summers at his home in Tuftonboro.
Smith is the third Republican to announce a bid to run against Shaheen, joining former state Sen. Jim Rubens and conservative activist Karen Testerman. However, state Republicans had expressed concern that Rubens and Testerman could be weak challengers to Shaheen and were looking for someone else, WMUR said.
Former Massachusetts U.S. Sen. Scott Brown seemed to be interested in entering the race, WMUR said, reporting that Brown was selling his Massachusetts home to live in New Hampshire full time.