SPRINGFIELD, Illinois (CNN) -- Presumptive Democratic presidential nominee Barack Obama introduced Sen. Joe Biden to the nation as his running mate Saturday, telling supporters that he is "a leader who is ready to step in and be president."

"For months, I've searched for a leader to finish this journey alongside me and join me in making Washington work for the American people. I searched for a leader who understood the rising costs confronting working people and will always put their dreams first," Obama said.

"Today, I've come back to Springfield to tell you I've found that leader," he said.

The rally is the pair's first joint appearance since Obama announced that Biden, the senior U.S. senator from Delaware, would be his running mate on his Web site and in a text message to supporters early Saturday morning. Watch Obama introduce his running mate »

Democrats hope that Biden's working-class roots and foreign policy experience will help Obama, who informed Biden of his decision Thursday.

Thousands of cheering supporters gathered Saturday for the rally in Springfield, Illinois, where Obama announced his candidacy last year. It will be their only public appearance together before next week's Democratic convention in Denver. Watch what Biden would bring to an Obama presidency »

As he took to the podium at Saturday's rally, Biden invoked the 16th U.S. president, Abraham Lincoln, who was from Illinois.

"President Lincoln once instructed us to be sure to put your feet in the right place and then stand firm," said Biden, a native of Scranton, Pennsylvania.

"Today in Springfield, I know my feet are in the right place, and I'm proud to stand firm with the next president of the United States of America, Barack Obama."

Obama stressed those roots, introducing 65-year-old Biden as "still that scrappy kid from Scranton who beat the odds."

He noted hardships in Biden's life, including the deaths of Biden's first wife and infant daughter in a 1972 car wreck and his struggle to overcome a brain aneurysm in 1988. iReport.com: iReporters ask if Biden was really the best choice

"That's the kind of fighter I want by my side in the months and years to come," Obama said.

Known for his plain-spoken approach and penchant for speaking from the cuff, Biden wasted little time taking to the traditional vice presidential candidate's role of political attack dog.

"John McCain ... served our country with extreme courage, and I know he wants to do right by America," he said of his Senate colleague and the presumptive Republican presidential nominee. "But the harsh truth is loud and clear: You can't change America when you supported George Bush's policies 95 percent of the time."

He also seized on a McCain comment from this week when he could not remember how many houses he owns and said McCain was out of touch with the "kitchen-table" issues that working families face.

"It's a pretty hard experience. He'll have to figure out which of the seven kitchen tables to sit at," Biden said.

The McCain campaign quickly pounced Obama for introducing Biden as "the next president of the United States" before quickly correcting himself to say "vice president." Watch McCain's new ad targeting Biden »

"Barack Obama sounded as though he turned over the top spot on the ticket today to his new mentor, when he introduced Joe Biden as the next president," McCain spokesman Ben Porritt said. "The reality is that nothing has changed since Joe Biden first made his assessment that Barack Obama is not ready to lead. He wasn't ready then and he isn't ready now."

Biden, chairman of the Senate Foreign Relations Committee, brings decades of experience that could help counter GOP attacks on Obama's lack of experience in foreign policy.

Biden abandoned his own White House run after a poor showing in Iowa's first-in-the-nation caucuses. He also ran for the 1988 Democratic presidential nomination but dropped out after charges of plagiarism in a stump speech. Learn more about Biden

Biden is serving out his sixth term, making him Delaware's longest-serving senator.

He is married and has three children. He received his undergraduate degree from the University of Delaware and got a law degree from Syracuse.

One of Biden's grandfathers was a Pennsylvania state senator, according to the Almanac of American Politics.

Biden will make his first big speech as the vice presidential candidate Wednesday, the third night of the Democratic convention.

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