Mandy Grunwald, an adviser to the Clintons for over two decades and a top strategist to Senator Elizabeth Warren of Massachusetts, is expected to serve as a senior adviser for communications to Hillary Rodham Clinton’s all-but-certain presidential bid. Ms. Grunwald will advise Mrs. Clinton on strategy and make some of the television ads along with Jim Margolis, whose firm, GMMB, is expected to take the lead on producing and buying the commercials. The pollster Joel Benenson is another senior adviser poised to work for Mrs. Clinton on her second White House run.