PITTSBURGH — Democratic running mates Barack Obama and Joseph Biden embarked on a Rust Belt bus tour yesterday that aimed to capitalize on a convention that appeared to ease party tensions and ended with Obama's historic and widely acclaimed acceptance speech before a record national audience.

But some of the Democrats' momentum quickly disappeared yesterday morning, when the campaign was caught off guard by the announcement of presumptive GOP nominee John McCain that he had selected Alaska Gov. Sarah Palin as his running mate. Campaign officials searched the Internet and debriefed Alaska Democrats to learn more about their mysterious foe, while a spokesman released a harsh initial statement about Palin that Obama later backed away from.

The Obama campaign, like most political observers, had expected McCain to select a more traditional running mate, such as former Massachusetts Gov. Mitt Romney or Minnesota Gov. Tim Pawlenty.

"I haven't met her before," Obama said at a biodiesel plant in Monaca, Pa. Nor had Biden, who will debate Palin in St. Louis on Oct. 8. Later yesterday afternoon, Obama spoke with Palin by telephone and wished her luck — though not too much luck — in the race. Biden also called Palin, and the two shared tales of the lengthy and secretive processes that led to their selections, aides to Biden said.

Obama described Palin as "a compelling person ... with a terrific personal story. I'm sure that she will help make the case for Republicans." But, he added, hitting on a theme from his convention speech, "Ultimately, John McCain is at the top of the ticket. He wants to take the country in the wrong direction. I'm assuming Gov. Palin agrees with him and his policies."

The Democratic team will spend Labor Day weekend on a caravan through the critical swing states of Pennsylvania, Ohio and Michigan, where the candidates will court working-class voters. Biden, a native of Scranton, Pa., with a lunch-pail pedigree, is expected to bolster Obama's credentials among this critical demographic, as well as with senior citizens.

Today, Obama and Biden will attend the funeral of Ohio Rep. Stephanie Tubbs Jones, who died suddenly Aug. 20. The senators will make stops in Dublin and Toledo, Ohio, and in Battle Creek, Mich. On Labor Day, Obama will march in a Detroit parade before speaking to an audience of union organizers who plan to mobilize for the Democrat.

Obama's acceptance speech at Denver's Invesco Field on Thursday night drew more than 38 million viewers, more than the opening ceremony of the Olympics and the most ever for a convention address.

Yesterday McCain and Palin made a direct appeal to supporters of Hillary Clinton, hoping to tap into lingering discontent with Obama among women voters. Mindful of this threat, Obama acknowledged Palin's ascent as the first woman named to a Republican presidential ticket, calling it "one more indicator of this country moving forward ... one more hit against that glass ceiling."

An initial campaign statement was less gracious. Obama spokesman Bill Burton noted Palin's staunch opposition to abortion and support for expanded oil drilling, and he ridiculed her resume — echoing the main argument McCain has directed at Obama.

Palin is in her first term as Alaska governor after serving as a council member and mayor of the small town of Wasilla. "Today, John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency," Burton said in the statement.

Republicans jumped on Burton's words as overly harsh, and the campaign soon produced a new statement credited to Obama and Biden that said: "We send our congratulations to Gov. Sarah Palin and her family on her designation as the Republican nominee for vice president. It is yet another encouraging sign that old barriers are falling in our politics. While we obviously have differences over how best to lead this country forward, Gov. Palin is an admirable person who will add a compelling new voice to this campaign."

At the biodiesel plant, Obama called his campaign's initial reaction a "hair-trigger" response, and said the latter statement by the two candidates is the one that "reflects our sentiments."