Washington - Democratic presidential hopeful Barack Obama on Saturday praised Joe Biden, his pick for vice president, as a man capable of changing the culture of politics in Washington despite his 36 years in the national legislature.

"For decades, he's brought change to Washington but Washington hasn't changed him," Obama said, introducing Biden at their first joint rally in Springfield, Illinois. "He's uniquely suited to be my partner as we work to bring our country back on track."

Obama's campaign had revealed the veteran Delaware Senator Biden - one of the party's leading voices on foreign policy - as his running mate in the early hours of Saturday morning in a text message sent to supporters.

Biden described his working class roots growing up in Scranton, Pennsylvania and told supporters he had the same "kitchen table" discussions about hard economic times as everyone else. Obama's campaign hopes that Biden's history will help with blue-collar Americans who have been slow to warm to the Democrat's campaign.

Biden praised Obama's Republican rival John McCain as a friend who "wants to do right by America," but lashed out at the Arizona senator for aligning himself with the policies of President George W Bush.

"You can't change America when you know your first four years as president will look exactly like the last eight years of George Bush's presidency," Biden said.

The Obama-Biden Democratic ticket made its first public appearance outside the state house in Springfield, where Obama got his political start in the state legislature and launched his presidential bid last February.

Obama's announcement of Biden as running mate ended weeks of speculation. US media broke the story only about three hours before the official text messages and email were sent out, a sign of just how closely guarded Obama's campaign had managed to keep the secret.

Biden, 65, is a longtime senator from Delaware and chairman of the upper house's Foreign Relations Committee. He is an expert in international relations who travelled to Georgia this month in the middle of the country's conflict with Russia, and will likely appeal to voters sceptical of Obama's foreign affairs credentials.

"Joe Biden is what so many others pretend to be: A statesman with sound judgement, who doesn't have to hide behind bluster to keep America strong," Obama said.

But Biden will come under fire for some comments made about Obama during his own brief presidential run this year. Biden questioned whether the 47-year-old Illinois senator was experienced enough, arguing the presidency was not "on-the-job training."

McCain called Biden Saturday morning to congratulate his long-time Senate colleague on being picked, even as his campaign released a ready-made television advertisement that used Biden's own comments critical of Obama.

"There has been no harsher critic of Barack Obama's lack of experience than Joe Biden," Ben Porritt, a spokesman for Republican rival John McCain, said in a statement.

Biden alluded to his reputation as a Washington insider in his Springfield speech, arguing that times had changed since he first entered the Senate in 1972.

"In all my years in the Senate, I have never ever in my life seen Washington so broken," Biden said, praising Obama as a pragmatist who was best able to end the partisanship that had prevented progress in Washington.

"There's something about this guy that allows him to bring people together like no one I've ever seen," Biden said.

Even for some McCain supporters, the addition of Biden and his foreign policy expertise to the Democratic ticket brought a sense of relief.

"I'm for McCain but I think Obama is going to win because of the Bush-Cheney problem," said Kendall Johnson, 69, a Denver businessman who is disappointed in Bush. "I want the next president and vice president to be as strong as possible."

The Democrats gather at their party convention in Denver, Colorado, beginning Monday to formally nominate Obama. Biden is slated to speak Wednesday night.

Praise for Biden came in from his Senate colleagues throughout the day, including from New York Senator Hillary Clinton, whose own supporters may be disappointed the former first lady was not chosen herself after an epic primary battle with Obama for the party nomination.

One of Biden's possible drawbacks is an outspokenness that has gotten him into trouble in the past, but it was Obama who committed the first minor gaffe for the new Democratic ticket when he introduced his running mate.

"Let me introduce to you the next president - the next vice president - of the United States of America, Joe Biden," he said.