Senator Joseph R. Biden Jr. of Delaware accepted the Democratic vice-presidential nomination on Wednesday night with an ode to his middle-class upbringing and a blistering attack on Senator John McCain.

“Again and again,” he said, “on the most important national security issues of our time, John McCain was wrong, and Barack Obama was proven right.”

He said Mr. McCain had supported President Bush’s policies that Mr. Biden said had driven the American economy into a ditch and backed Mr. Bush on a war that is costing American taxpayers $12 billion a month.

As Democrats often do, he paid tribute to Mr. McCain’s military service and his more than five years as a prisoner of war in Vietnam. That is not sufficient qualification for the presidency, Mr. Biden said in as direct a way as any Democrat this year has.

Gov. Sarah Palin of Alaska introduced herself to America before a roaring crowd at the Republican National Convention on Wednesday night as “just your average hockey mom” who was as qualified as the Democratic nominee, Senator Barack Obama, to be president of the United States.

An hour later Senator John McCain, a scrappy, rebellious former prisoner of war in Vietnam whose campaign was resurrected from near-death a year ago, was nominated by the Republican Party to be the 44th president of the United States after asking the cheering delegates, “Do you think we made the right choice” in picking Ms. Palin as the vice-presidential nominee.