Brown spoke Friday afternoon, in a hotel in Nashua, N.H., the New York Times reported. He emphasized his New Hampshire ties and attacked President Obama's healthcare law and Sen. Jeanne Shaheen's support for it.
In early 2010, Brown, a Massachusetts state legislator, became a national political star when he won a special election to succeed Sen. Ted Kennedy, D-Mass., the first Republican elected to the Senate from the state since 1972. In 2012, however, he lost to Elizabeth Warren.
Shaheen, a Democrat, was elected to the Senate in 2008 after serving as governor of New Hampshire. A recent poll by Suffolk University in Boston gave her 52 percent of the vote to 39 percent for Brown with 9 percent undecided, the Boston Globe reported.
In his speech, Brown said the healthcare system in the United States was "very good" before the passage of the Affordable Care Act.
"A big political wave is about to break in America and the Obamacare Democrats are on the wrong side of that wave," Brown said.
Brown, who moved to New Hampshire last year, also preemptively defended himself from suggestions he is a carpetbagger. He spoke of childhood visits to his grandparents in New Hampshire and said he has put in close to 300,000 miles on his pickup truck driving around the state.