An investigation by the New York State Board of Elections found evidence of flagrant violations of campaign finance law by a team of people that Mayor Bill de Blasio created to raise money for Democrats running for the State Senate in 2014, according to a confidential board report. The board sent the report to Cyrus R. Vance Jr., the Manhattan district attorney, three months ago, prompting a criminal investigation into the fund-raising. The mayor’s Senate effort, which failed in its goal to wrest the legislative body from Republican control, has proved damaging to him in multiple ways. Democrats remained in...