DENVER — Sen. Barack Obama moved to deal with two potential weaknesses when he named Sen. Joseph R. Biden Jr. as his vice presidential running mate Saturday. Biden shores up Obama's inexperience on national security issues and brings to the ticket a politician eager to lead the attack against Republican Sen. John McCain.

The choice of Biden could produce another benefit for Obama and the Democrats. Biden's opening speech at a joint appearance with Obama Saturday in Springfield, Ill., not only included tough words about McCain but also underscored his potential to sharpen the campaign's message on middle-class economic issues that are paramount with the electorate.

But the voluble Biden is not a risk-free running mate for Obama. Republicans were quick to remind people that during the Democratic nomination battle, Biden described Obama as not ready to be president. More significantly, they also see Biden as a potential gaffe machine likely to distract attention from Obama. That view, stated more gently, was shared privately even by some Democrats on Saturday as they assessed a choice that in almost all other respects drew widespread praise within the party.

The die may have been cast for Biden when Russian forces invaded Georgia earlier this month. Until then, Obama may have believed he had more latitude in his choice, that he could worry less about dealing with his perceived weaknesses and instead pick a running mate who would more clearly buttress the change and generational messages at the heart of his candidacy.

Once the tanks rolled, the weight of evidence shifted toward someone who would raise no questions in the area of national security. The "first do no harm" criterion in vice presidential selections became vital. Among those under serious consideration, the 65-year-old Biden was at the top of the list on national security credentials.

Biden also has a powerful personal story, and the Obama campaign was quick to try to capitalize on it. In parallel speeches, Obama described Biden as a symbol of the American Dream, and Biden then returned the favor for a politician whose biracial heritage and unusual biography have left some voters uncertain about who he is.

"Barack and I come from very different places," he said, "but we share a common story, an American story."

That is a theme likely to become more and more prominent.

His long service in the Senate also has included time as chairman of the Judiciary Committee, giving him experience in judicial appointments and crime legislation. He has strong ties with organized labor, which does not know Obama nearly as well.