Former Republican Sen. Scott Brown of Massachusetts said Friday he was preparing to run for a seat in neighboring New Hampshire, likely forcing Democrats to devote money to an additional state as they try to retain their majority in the Senate this fall.

Mr. Brown ended months of speculation Friday by announcing he had launched an exploratory committee to challenge Democratic Sen. Jeanne Shaheen. The move allows Mr. Brown to raise money and hire staff as he gears up for what promises to be one of the year's marquee contests.

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