MINEOLA, N.Y. — Thomas R. Suozzi conceded defeat on Tuesday in the race for the Nassau County executive, a harsh setback for Mr. Suozzi, the Democratic incumbent who was widely regarded as a potential candidate for higher office. He acknowledged his loss after a recount of the votes cast last month showed that his Republican challenger, Edward P. Mangano, a county legislator, had a 386-vote lead. Mr. Suozzi, speaking to reporters at a news conference, said he believed it was “mathematically impossible to win. I didn’t think it was fair to the public, my team or Mr. Mangano, and I...