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 a 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 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 Scanton, Pennsylvania, telling supporters he had the same "kitchen table" discussions about hard economic times as everyone else.

He 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 over which of a handful of likely politicians would get the nod. 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.

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 reportedly 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, arguing that times had changed since he first entered the Senate in 1972.

"In all my years in the Senate, I have never ever seen Washington so broken," Biden said. "These times call for a total change in Washington's world view."

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.

Clinton, who addresses the convention herself on Tuesday, called Biden an "exceptionally strong, experienced leader and devoted public servant." (dpa)