That nomination will formally come later Wednesday as delegates to the Democratic National Convention crown Obama as the first black nominee of a major political party.
Obama was due to arrive in the convention city Wednesday afternoon.
Former President Clinton was also set to deliver a prime-time address at the convention, a day after his wife, Hillary Rodham Clinton, sought to unify the party after a bitter primary season, delivering the second part of a one-two punch.
Representatives of Clinton and Obama struck a deal setting ground rules for Wednesday's roll call vote that will hand the nomination to Obama, but will also allow Clinton supporters to express their support for her.
Advisers to Clinton and Obama sent a joint letter to state delegation chairs instructing them to distribute vote tally sheets to delegates Wednesday and return them by before the vote is scheduled to get under way at 6 p.m. EDT.
The letter, first obtained Tuesday night by The Associated Press, said Clinton would have one nominating speech and two seconding speeches, followed by Obama's nominating speech and three seconding speeches - totaling no more than 15 minutes for each candidate. Then the roll call will begin, said the letter signed by Obama senior adviser Jeff Berman, Clinton senior adviser Craig Smith and convention secretary Alice Germond.
The roll call will continue until all votes are counted or someone asks the delegates to give the nomination to Obama by acclamation.
Democratic officials close to Clinton say they plan to have someone - perhaps the senator herself - cut off the vote after a few states.
Anticipating Wednesday night's focus on national security at the Democratic National Convention, Republican John McCain contended in a new TV ad that Obama showed he was "dangerously unprepared" for the White House when he described Iran as a "tiny" nation that didn't pose a serious threat.
"Iran. Radical Islamic government. Known sponsors of terrorism. Developing nuclear capabilities to 'generate power' but threatening to eliminate Israel," says the ad, which was being run in key states. "Terrorism, destroying Israel - those aren't 'serious threats"'?
Missing from the ad was the context of Obama's remarks last May in which he compared Iran and other adversarial governments to the superpower Soviet Union. "They don't pose a serious threat to us the way the Soviet Union posed a threat to us," he said in arguing for talks with Iran. "You know, Iran, they spend one-100th of what we spend on the military. If Iran ever tried to pose a serious threat to us, they wouldn't stand a chance."
The McCain ad signaled a shift from trying to stir up Hillary Clinton's supporters with her primary-season criticism of Obama to raising fears about Obama's ability to handle international threats. Clinton closed the book on her 2008 presidential bid Tuesday night with an emphatic plea for the party to unite behind Barack Obama.
The Democratic convention spotlight was turning to her husband, the former president, as he prepared to take the prime-time television stage Wednesday night. He is expected to launch attacks on McCain and on the Bush administration, particularly on the state of the U.S. economy.
Hillary Clinton, who won 18 million votes but still failed to earn her party's nomination, planned to meet with delegates who still want to cast ballots for her during the nominating roll call Wednesday evening.
Clinton's aides said it remained unclear how exactly the meeting with the delegates would play out, or how her supporters will react.
"It's not Hillary's job to bring this party together," said Jennie Lou Leeder, a Clinton delegate from Llado, Texas. "It's Barack Obama's job to bring this party together."
It's the kind of talk that Clinton tried to discourage. "I want you to ask yourselves: Were you in this campaign just for me?" she said Tuesday night in her convention speech, addressing her supporters.
Clinton used herto try to silence infighting over how to honor Clinton's campaign without distracting from Obama's upcoming contest against McCain.
"Barack Obama is my candidate, and he must be our president," she said.
Even so, bringing the Democratic Party together is going to take more than a single speech. The best unifier among Democrats going into the final sprint might just be McCain.
"Arizonans are also proud of their political tradition, from Barry Goldwater to Mo Udall to Bruce Babbitt. There's a pattern here," Arizona Gov. Janet Napolitano told delegates Tuesday as part of the chorus eviscerating McCain. Goldwater, Udall and Babbitt all sought the presidency; none succeeded.
"Speaking for myself, and for at least this coming election, this is one Arizona tradition I'd like to see continue," Napolitano said.
Republicans, meanwhile, struggled for a bit of the spotlight. On Wednesday, former New York Mayor Rudy Giuliani, the keynote speaker for the Republican convention next week, said that Hillary Clinton never told delegates that Obama was prepared for the presidency.
"Nowhere in that speech did she answer the question about his character, his ability to lead, the things that are at issue here," Giuliani said on CBS News' The Early Show. "And until she does, you're going to have a lot of Hillary Clinton supporters that are either not going to vote ... or are going to vote for John McCain."
Former Massachusetts Gov. Mitt Romney, a potential running mate for McCain, also came to Denver and said Tuesday, "Barack Obama is a charming and fine person with a lovely family, but he's not ready to be president."
Bill Clinton, whose reputation took some hits during the primary season, stayed away from his wife and daughter Chelsea - who introduced her mother on stage Tuesday evening. Instead, he watched his wife's speech from convention floor box seats.
"She was great," Clinton told The Associated Press as he left the convention hall. "Weren't you proud of her?"
Obama, 47, formally receives the nomination Wednesday. He delivers his acceptance speech Thursday night at a football stadium. An estimated 75,000 tickets have been distributed for the event, meant to stir comparisons with John F. Kennedy's appearance at the Los Angeles Coliseum in 1960.
McCain and his yet-unnamed running mate are scheduled to receive their nomination at the Republican convention in Minneapolis next week.