While Mr. Bolton has left plenty of bruises over the years and he was accused of politicizing intelligence before the Iraq war, an accusation he denied, many former colleagues said he was forthcoming to a fault.

“Ambassador Bolton was known for being frank and candid — in fact, some complained that his frankness and candor got in the way of a more indirect, diplomatic approach,” said Peter D. Feaver, a national security aide in Mr. Bush’s White House. “I do not recall ever hearing people complain that he made up stuff.”

Democrats at this point are happy to vouch for his honesty. “I think Bolton has a lot of credibility,” Senator Thomas R. Carper of Delaware, who once called him “too extreme,” said on Fox News this week. “Not just among the Republicans but our side as well.”

On the other hand, Senator Rand Paul, Republican of Kentucky, said Mr. Bolton was not to be believed. “I would say that he’s a witness very interested in making a lot of money right now,” Mr. Paul said on CNN. “A month ago, he was against testifying. Now that his book is complete and available for $29.95, he’s all for testifying. So I think we need to take with a grain of salt his testimony if he comes in.”