ST. PAUL -- Democratic presidential candidate Barack Obama begins airing an ad Saturday that responds to rival John McCain's selection of a running mate, carefully avoiding any direct criticism of Sarah Palin, the Alaska governor whom McCain chose for the GOP ticket.

Obama's campaign promptly created the spot in advance of next week's Republican National Convention. The ad, called "No Change," sought to sustain the theme that Obama and Democrats worked to cultivate at their own convention this week -- that McCain represents a continuation of the policies of an unpopular President Bush.

"Well, he's made his choice," the ad states. "But, for the rest of us there's still no change. McCain doesn't get it, calling this broken economy 'strong.' Wants to keep spending ten-billion-a-month in Iraq. And votes with George Bush 90 percent of the time."

The ad continues: "So, while this may be his running mate..." as an image of McCain and Palin appears on the screen. The image then shifts to a shot of McCain with Bush. "America knows this is John McCain's agenda. And we can't afford four more years of the same."

The ad illustrates the Obama camp's careful reaction to Palin's addition to the Republican ticket -- a groundbreaking move that reintroduced gender into the presidential race just as the Democrats worked to resolve a rift between the Obama camp and supporters of Hillary Rodham Clinton.

Obama and his running mate, Joe Biden, both called Palin to congratulate her on Friday. They also issued a joint statement that said her selection was "yet another encouraging sign that old barriers are falling in our politics." They called her "an admirable person and (she) will add a compelling new voice to this campaign."

Earlier, however, Obama spokesman Bill Burton issued a statement calling Palin an abortion-rights opponent and "the former mayor of a town of 9,000 with zero foreign policy experience."

Obama, asked later about the message disparity, said: "I think, you know, campaigns start getting these hair triggers. And the statement that Joe and I put out reflects our sentiments."

As for the content of the ad, the reference to McCain "calling this broken economy strong" oversimplifies what McCain has said. In a recent radio interview, McCain said the "the fundamentals of our economy are strong." But he went on to say: "We've got terribly big challenges now, whether it be housing or employment. ... It's very, very tough times. It's very tough. But we're still the most innovative, the most productive, the greatest exporter, the greatest importer."

The Obama spot comes as the McCain campaign ramps up its own advertising.

McCain and the Republican National Committee planned to begin airing a new ad that depicts Obama as nothing more than an accomplished speaker, complete with scenes of his speech last month before a huge outdoor crowd in Berlin. The ad will run in his 11 key battleground states, but he also is adding Minnesota, at least during the Republican National Convention, which opens here Monday.