The race to fill the state Senate seat vacated by new US Senator Scott Brown is shaping up to be a four-person contest.

The latest candidate to raise his hand is Richard Mitchell, a 47-year-old computer engineer at EMC who moved from Sherborn to Natick in 2007. He said he is running to advocate for fathers’ rights.

“I’ve gone through the family and probate court system and it’s a compete mess and something has to be done about it,’’ he said in a phone interview yesterday. “That is the single reason why I’m running.’’

He is a registered independent, and this is his first time seeking political office.

Candidates who had previously announced are state Representative Lida Harkins, a Needham Democrat; state Representative Richard Ross, a Wrentham Republican; and Peter Smulowitz, a Needham physician and a Democrat who will face off against Harkins in the primary.

Before yesterday’s 5 p.m. deadline for submitting paperwork to local election authorities, all four candidates had filed voters’ signatures with town clerk offices. Clerks, or other election authorities, must certify at least 300 signatures from registered voters for each candidate. The candidates have until 5 p.m. March 9 to submit the certified signatures to the state Elections Division. The state primary date is April 13, and the election is May 11.

The Norfolk, Bristol, and Middlesex state Senate seat represents Millis, Needham, Norfolk, North Attleborough, Plainville, Sherborn, Wayland, and Wrentham as well as parts of Attleboro, Franklin, Natick, and Wellesley.

Harkins, who is 66, was first elected state representative in 1988, and has served as majority whip and as chair of the Joint Committee on Education, Arts, and the Humanities; the Joint Committee on Housing and Urban Development; and the House Committee on Science and Technology.

Ross, 55, succeeded Brown as Wrentham selectman and then as state representative, an office Ross has held since 2005. Ross, who runs a funeral home, has made health care a key issue, saying that the new state mandate on health coverage is too tough on small businesses.

Smulowitz, a 33-year-old emergency room physician at Beth Israel Deaconess Hospital-Needham and Beth Israel Deaconess Medical Center, is pledging a fresh perspective. He was a founding member of Needham for Barack Obama and is a member of Needham Town Meeting.

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