“This is more evidence that Obama’s politics of hope is just empty words,” Mr. Conant said in a statement.

Mr. McCain’s choice of Ms. Palin  and the resulting jolt of energy among Republican voters  appears to have caught Mr. Obama and his advisers by surprise and added to concern among some Democrats that the Obama campaign was not pushing back aggressively enough against Republican attacks in a crucial phase of the race.

“Our ads have been pretty tough. I just have different philosophy: I’m going to respond with the truth,” Mr. Obama told a voter on Friday, responding to a question about whether Democrats would suffer the same fate they have in previous presidential campaigns. “I know there are a lot of Democrats and some independents and some Republicans who really want change and are getting really nervous because they have seen this movie before.”

The new tone is also to be presented in television interviews with local stations in five swing states, backed up by the new ads and appearances throughout the country by supporters. Anotheradvertising theme will be pay equity for women, an issue that has particular resonance as the campaigns battle for female voters.

Despite the shift in intensity, Mr. Obama’s aides said they were confident with the course of the campaign. They said that, other than making some alterations around the edges, particularly in response to Mr. McCain’s effort to seize the change issue from Mr. Obama, they were not planning any major deviation from a strategy that called for a steady escalation of attacks on Mr. McCain as the race heads toward the debates.