Mr. Klain has gone in and out of government over the past several decades, at times practicing as a lawyer and later working with Steve Case, the founder of AOL, in a venture capital investment firm called Revolution. His initial agreement with Mr. Case in 2005 included the right to take unpaid leave in September and October every four years to participate in presidential politics.
“He can process a lot of information, focus on the things that matter and balance a lot of balls,” Mr. Case said of Mr. Klain, calling him “unflappable” and pragmatic when it comes to confronting a challenge: “‘Here’s the problem or the opportunity. Let’s narrow in on what matters and get the right team in place.’”
Mr. Case dismissed concerns from some people that Mr. Klain was too much of a Washington insider for this moment in history, noting that as an Indiana native, Mr. Klain often goes home for the Indianapolis 500.
“He’s been in Washington a long time,” Mr. Case said. But he added, “He’s an Indiana guy.”
Over his years mingling among the nation’s top Democratic politicians, Mr. Klain has become a specialist in helping presidents win confirmation battles in the Senate. During Mr. Obama’s first term, he played a central role in helping win confirmation of Justices Sonia Sotomayor and Elena Kagan to the Supreme Court.
He also developed a reputation as the Democratic Party’s expert on debate preparation, coaching virtually all of the party’s presidential and vice-presidential nominees during the last two decades. He made little secret of his frustration with sitting presidents — including Mr. Obama — complaining that they too often refused to prepare.
A memo for his clients that Mr. Klain prepared years ago has become memorialized as the party’s best advice for its candidates. Among the 21 rules that he outlined: No. 10, “Punches are good, counterpunches are better”; No. 13, which says that a candidate can lose a debate at any time but that “you can only win it in the first 30 minutes”; and No. 20, which warns not to say something that does not feel right.
“At that moment remember the advice that some elementary schoolteacher once gave you: ‘If in doubt, don’t,’” Mr. Klain wrote.