As chief strategist for Hillary Clinton’s unsuccessful 2008 campaign for president, he conceived the “3 a.m.” commercial that raised doubts about whether Barack Obama, then a senator, was ready for the Oval Office. Mr. Penn argued in an essay he wrote for Time magazine in May that “negative ads are, by and large, good for our democracy.”

But his approach has ended up souring many of his professional relationships. He left Mrs. Clinton’s campaign after an uproar about his consulting work for the government of Colombia, which was seeking the passage of a trade treaty with the United States that Mrs. Clinton, then a senator, opposed.

“Google should be prepared for everything but the kitchen sink thrown at them,” said a former colleague who worked closely with Mr. Penn in politics and spoke on condition of anonymity. “Actually, they should be prepared for the kitchen sink to be thrown at them, too.”

Hiring Mr. Penn demonstrates how seriously Microsoft is taking this fight, said Michael A. Cusumano, a business professor at M.I.T. who co-wrote a book about Microsoft’s browser war.

“They’re pulling out all the stops to do whatever they can to halt Google’s advance, just as their competition did to them,” Professor Cusumano said. “I suppose that if Microsoft can actually put a doubt in people’s mind that Google isn’t unbiased and has become some kind of evil empire, they might very well get results.”

At Microsoft, Mr. Penn has assembled a team of fewer than a dozen people, about two-thirds of whom operate out of offices in Washington, where Mr. Penn lives, with the rest in the Seattle area. It includes some of his lieutenants from politics and from Burson-Marsteller, the public relations firm where he was chief executive before joining Microsoft, a Burson client. Josh Gottheimer, a former senior counselor to the chairman of the Federal Communications Commission, is a recent addition.

“The concept is to create a SWAT team to work with the product teams on some of their toughest problems,” Mr. Penn said.