(See Corrections and Amplifications item below.)

DENVER -- Barack Obama's choice of Delaware Sen. Joe Biden as his running mate heightened tensions between the Democratic candidate and supporters of former rival Hillary Clinton, giving Republican John McCain an opening to woo these voters.

While most Clinton delegates had anticipated Sen. Obama wouldn't choose the former first lady as a running mate, his first joint appearance Saturday with Sen. Biden in Springfield, Ill., left many of them frustrated.

"There's a lot of pain that needs to be addressed," said Laura Boyd, a Clinton delegate in Oklahoma. Ms. Boyd said she held out hope Sen. Clinton might have been chosen.

During the long Democratic primary fight, Sen. Obama had said Sen. Clinton would be "on anyone's short list" for vice president. But reports last week that Sen. Obama's vice-presidential-search team hadn't vetted the New York senator rankled many Clinton backers.

When asked why Sen. Obama didn't choose Sen. Clinton, senior Obama strategist David Axelrod told ABC News Sen. Clinton is "going to be an important, an important voice in this campaign" but that Sen. Obama felt Sen. Biden would be the "best fit for him at this time."

Obama spokeswoman Jen Psaki said Sen. Obama had a "lengthy" conversation with former President Bill Clinton Thursday about a range of campaign issues. He didn't tell Mr. Clinton about his vice-presidential choice.

Sens. Obama and Clinton spoke Friday, according to Ms. Psaki, who declined to say whether the two discussed Sen. Obama's vice-presidential choice.

The Obama-Biden ticket could create an awkward dynamic at the Democratic National Convention, which begins Monday. Nearly 2,000 Clinton delegates flooded into Denver Sunday, some of them wearing "Women for Hillary" buttons. A large number of these delegates plan to cast a symbolic roll-call vote for Sen. Clinton Wednesday, the same day Mr. Clinton and Sen. Biden will deliver speeches.

Sen. Clinton is scheduled to speak at the convention Tuesday.

Sen. McCain has tried to take advantage of any lingering tensions in an effort to win over the 18 million people who voted for Sen. Clinton in the prolonged Democratic primaries.

A Wall Street Journal/NBC News poll found 52% of respondents who voted for Sen. Clinton in the primaries plan to vote for Sen. Obama and 21% plan to vote for Sen. McCain.

"She inspired young women and people all over America and I think she deserves credit for running a very good campaign," Sen. McCain told a Clinton supporter during a recent town-hall meeting in York, Pa.

Sunday morning, the McCain campaign began airing an ad questioning why Sen. Obama didn't choose Sen. Clinton as his running mate.

"She won millions of votes. But isn't on the ticket? Why?" an announcer says in the 30-second spot titled "Passed Over." The ad ticks off a litany of attacks Sen. Clinton used against Sen. Obama in the primary contest. "For speaking the truth," the narrator concludes.

"[Sen. Clinton] has said repeatedly that Barack Obama and she share a commitment to changing the direction of the country, getting us out of Iraq and expanding access to health care," said Clinton spokeswoman Kathleen Strand. "It's interesting how those remarks didn't make it into his ad."

Sen. Obama has recently inserted a more populist tone into his stump speeches, an attempt to better connect to the white, working-class voters who largely backed Sen. Clinton during the primaries.

On Saturday, he touted Sen. Biden's working-class roots and told the crowd repeatedly that Sen. Biden lives in Wilmington, Del., and takes the train to Washington for Senate business. "This working-class kid from Scranton and Wilmington has always been a friend to the underdog," Sen. Obama said to the 35,000 people gathered outside the Old State Capital in Springfield.

But some Democratic strategists say Sen. Clinton, whose father was from Scranton, Pa., and who established herself as an advocate for the working class during the Democratic primaries, could have given Sen. Obama an instant connection to these voters.

"If Barack Obama wants a fighter who has wrestled" with health care, the economy and other kitchen-table economic issues, then "he could do no better" than Sen. Clinton, said Erika Alexander, a Clinton delegate from California.

Write to Amy Chozick at amy.chozick@wsj.com

Corrections and Amplifications:

A transcript supplied by Sen. Barack Obama's campaign of his speech Saturday to introduce his running mate referred to Sen. Joe Biden as a "working-class kid from Scranton and Wilmington." However, when Sen. Obama delivered the speech, he referred to Sen. Biden as a "working-class kid from Scranton and Washington." This article referred to the comment as written in the transcript, but failed to note the change in the delivered version.