Former Gov. Deval Patrick of Massachusetts officially entered the presidential race on Thursday, adding an 18th candidate and a new twist to a turbulent Democratic primary with less than three months to go before the Iowa caucuses. “I admire and respect the candidates in the Democratic field,” Mr. Patrick said. “They bring a richness of ideas and experience and a depth of character that makes me proud to be a Democrat. But if the character of the candidates is an issue in every election, this time is about the character of the country.”