DENVER — Barack Obama launched his historic fall campaign for the White House on Thursday with an outdoor Democratic National Convention extravaganza that blended old-fashioned speechmaking, Hollywood-quality stagecraft and innovative, Internet age politics.

One day after becoming the first black man to win a major party presidential nomination, Obama readied the most important speech of his improbable candidacy, a prime-time address to an estimated 75,000 inside Denver’s NFL stadium and uncounted millions watching at home on television.

Aides pledged a direct conversation with voters about the choice between Obama, a 47-year-old Illinois senator, and his Republican rival, Sen. John McCain of Arizona.

In an audacious move, McCain worked to steal at least a portion of the political spotlight by stoking speculation that his selection of a vice presidential running mate was imminent. An aide said McCain had made his decision, and one man on the short list, Gov. Tim Pawlenty of Minnesota, canceled all public appearances, raising attention even higher.

McCain is expected to announce his pick soon and appear with the person at a rally in Dayton, Ohio, on Friday.

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