Barack Obama launched his historic campaign for the White House with an outdoor extravaganza Thursday, promising more than 84,000 cheering supporters an end to the "broken politics in Washington and the failed policies of George W. Bush." In a nationally televised speech in which he accepted the Democratic presidential nomination, Obama pressed the themes that will guide his campaign through the Nov. 4 election: that he will help the country's struggling middle class and be strong on national security, and that a victory for rival John McCain would amount to a third term for Bush. "Tonight, I say to the people of America, to Democrats and Republicans and independents across this great land - enough!" he said. The speech capped the four-day Democratic National Convention, where he sought to unite a party divided after a bitter primary fight and regain momentum in the campaign against McCain.