State Senator Jarrett T. Barrios ’90 announced yesterday that he will withdraw from the campaign for Middlesex County district attorney and seek reelection to his current post.

Barrios’ decision leaves Gerry Leone ’85, a former Kirkland resident who helped win the 2003 conviction of “shoe bomber” Richard Reid, running unopposed to be the top law enforcement official in the county that comprises Cambridge.

It also sets the stage for a fierce Democratic primary contest in the state Senate district that includes parts of Harvard’s campus.

Barrios, who spoke in Kirkland last month as part of the Harvard College Democrats’ Massachusetts Politics Week, is likely to face at least five other candidates in the Senate race, including Cambridge City Councilor Anthony Galluccio, who also ran against him in the 2002 election.

Barrios wrote in an e-mail to his supporters that his campaign for district attorney has “imposed major burdens” on his family.

“I have no doubt that we would have been successful on Election Day,” Barrios wrote. “I simply could not justify the sacrifices involved in running for this office, when there is so much that I can continue to accomplish in the Senate.”

But longtime local politics observer Robert Winters said that Barrios’ withdrawal from the district attorney’s race “was his only choice. He had no chance against Gerry Leone.”

The current district attorney, Martha Coakley, has endorsed Leone’s bid. She is now running for state attorney general.

The 1,000 signatures required to appear on the ballot for county district attorney are due on May 2, and Winters said it was unlikely someone would join the race. Given the support Leone has acquired, it would be a “futile run,” Winters said.

Leone’s campaign announced yesterday that it had raised over $700,000 for the race.

In a statement on Barrios’ withdrawal yesterday, Leone said: “I understand and appreciate his very personal decision to step away from this race. Senator Barrios has done many good things for the people of his district and of this state, and I look forward to working with him in the future on important issues of public safety and justice.”

Back in the Senate, Barrios told supporters, the former Adams House resident will focus on “cracking down on domestic violence and identity theft, addressing bullying and violence in schools, and advancing progressive priorities on a variety of other issues.”

Barrios is the first openly gay Hispanic Massachusetts state senator.

Barrios garnered 48 percent to Galluccio’s 32 percent when the two squared off in the 2002 Democratic primary. Winters predicted that this year’s primary may be closer.

But Galluccio suffered a setback late last month when the Boston police said they are investigating allegations that he was drunk when a car he was driving crashed in December.

—Staff writer Claire M. Guehenno can be reached at guehenno@fas.harvard.edu.