Their stop in Cedarburg (where Ms. Palin ordered some richly mixed “moose tracks” ice cream) showed their determination to challenge Mr. Obama in Wisconsin, a state where he has enjoyed a comfortable edge but where Mr. McCain has invested heavily in television advertising.

Democratic strategists said Friday that they did not yet know whether the field of battleground states would expand or contract with the addition of Ms. Palin to the ticket. McCain advisers said they planned to use her in the strongly conservative areas where she has the most appeal. They also said her popularity ensured they would send her to large television markets as well as the small and medium markets traditionally reserved for the vice-presidential nominee.

“Whether she has additional reach,” said Mr. Axelrod, the Obama strategist, “is something that we’ll have to see.”

The Obama campaign’s immediate focus is on the economy and a broader call for change, but questions from voters suggest that the race could also turn on social and cultural matters. For the second straight day, Mr. Obama was asked whether he would tighten rules for gun owners, an issue that has proved vexing to many Democratic candidates.

“If you believe that I’m the best guy when it comes to jobs,” he replied, “if you believe that I’m the best guy when it comes to health care, if you believe that I’m the best guy on education and I’m going to be looking out for you and fighting for you, this can’t be the reason not to vote for me. Your guns  we’re not going to mess with them, all right?”

Ms. Palin, meanwhile, opened a new front against Mr. Obama, who said in an interview televised by Fox News on Thursday that the troop escalation in Iraq had succeeded beyond expectations.

“Just last night, Senator Obama finally broke and brought himself to admit what all the rest of us have known for quite some time,” Ms. Palin said in Cedarburg, adding that “thanks to the skill and valor of our troops, the surge in Iraq has succeeded.”