With 100 days remaining before Election Day, there is an air of apprehension around the Obama campaign headquarters here. Yet there are few regrets about the tone of the race, only a conviction that the circumstances — a frail economy, intense Republican opposition and a well-financed negative campaign from Mr. Romney and his allies — left Mr. Obama no option but to fight back even if it sullies his image as a candidate of hope and change…

Mr. Obama, whose competitive and confident streaks seem to have been rekindled by the challenge from Mr. Romney, has shown no inclination to hold back in trying to portray his rival as a secretive Bush-era throwback whose wealth puts him out of touch with the middle class…

His aides wondered in particular whether a recent spot showing Mr. Romney singing “America the Beautiful,” while headlines about outsourcing and overseas bank accounts flashed, was too negative. The president and some of his strategists were unsure, aides said, but a focus group of voters found it to be fair. They still tested the ad again before showing it…

While Mr. Obama rose through the ranks with a clarion call for a new kind of politics, there is little noticeable criticism about the tenor of the race from longtime supporters.