Barack Obama clinched the Democratic presidential nomination Tuesday, becoming the first African-American to run for the nation's highest office as the standard bearer of a major political party.

The Illinois senator won the final Democratic primary of the season in Montana Tuesday night, adding to the flood of superdelegates that put him over the 2,118 delegates it takes to become the nominee. Hillary Clinton won the South Dakota primary earlier in the evening, but it did not provide nearly enough delegates to push her over the top.

"This is our moment ... our time to turn the page on the policies of the past," Obama declared Tuesday evening in St. Paul, Minn., the site of this summer's Republican National Convention. The crowd was estimated at 17,000 inside the stadium and 15,000 more outdoors.

"Because of you, tonight I can stand here and say that I will be the Democratic nominee for the president of the United States of America," he said.

Obama kicked off his general election battle against presumptive GOP nominee John McCain, accusing the Arizona senator of wanting to prolong President Bush's "failed" economic and foreign policies. The charge is a likely theme for Obama in the months to come.

"While John McCain can legitimately tout moments of independence from his party in the past, such independence has not been the hallmark of his presidential campaign," Obama said. "It's not change when John McCain decided to stand with George Bush 95 percent of the time, as he did in the Senate last year."

The McCain campaign countered that Obama's record of partisanship is no different as he voted with his party more than 95 percent of the time. It added that by using Obama's own standard, he voted with the president 45 percent of the time in the past two years.

In his victory speech, Obama praised Clinton for making "history" with her campaign and for carrying "an unyielding desire to improve the lives of ordinary Americans, no matter how difficult the fight may be."

Clinton, speaking in New York City Tuesday evening, did not concede the race.

"This has been a long campaign, and I will be making no decision tonight," she said to cheering supporters. She said she would meet with party leaders and supporters to determine how to go forward.

In Montana, Obama won with 56 percent to Clinton's 41 percent. FOX News exit polls showed Obama beat Clinton in virtually every voting bloc.

Clinton handily won the South Dakota contest. She beat Obama by 55 to 45 percent.

Exit polls showed she ran especially well among elderly voters, female voters and voters who decided in the last three days.

Polls closed in the two final primaries exactly five months after Obama wowed the political establishment with his lead-off Iowa caucus victory. The Montana win allowed Obama to end on a high note after suffering some embarrassing losses to Clinton over the past two months.

But since the final states together were worth only 31 delegates, far more attention was paid Tuesday to the avalanche of superdelegates that supposedly fell into Obama's camp throughout the day.

As he clinched the nomination and won the Montana primary, the Obama campaign declared that it swept up 26.5 more superdelegate endorsements.

With some Montana and South Dakota pledged delegates still outstanding, the AP reported that Obama has 2,154 delegates, and Clinton has 1,919.

With Obama pivoting to the general election, McCain, who clinched his party's nomination three months ago, took his opening shot, declaring Tuesday that "the general election campaign has begun" and saying, "This is, indeed, a change election ... But the choice is between the right change and the wrong change, between going forward and going backward."

McCain held a prime time speech Tuesday night in the New Orleans suburb of Kenner, La., in what is essentially a kickoff to his fall campaign. The backdrop to his address was a new banner that read: "A Leader We Can Believe In" - a direct swipe at Obama's "Change We Can Believe In" slogan. He was introduced by Louisiana Gov. Bobby Jindal, an oft-mentioned potential running mate for McCain.

McCain, who spoke to a much smaller crowd than both Obama and Clinton, also deflected the charges that he is running for a third Bush term.

"Why does Senator Obama believe it's so important to repeat that idea over and over again?" McCain said. "Because he knows it's very difficult to get Americans to believe something they know is false. So he tries to drum it into your minds by constantly repeating it rather than debate honestly the very different directions he and I would take the country."

Clinton aides denied that the campaign would pack it in Tuesday, but notably left the door open for what would happen come Wednesday. Clinton also said on a conference call with New York lawmakers Tuesday that she is "open" to being Obama's running mate if it would help the party.

Advisers said she was withholding a formal departure from the race partly to use her remaining leverage to press for a spot on the ticket.

Sources familiar with the contents of the conference call told FOX News that there was a "sense of resignation" from Clinton. According to one participant on the call, Clinton acknowledged she did not have the delegate math to overtake Obama, but just wanted to take time to determine how to exit the race.

Clinton supporter and New York Rep. Charlie Rangel talked up the concept Tuesday of an Obama-Clinton ticket Tuesday afternoon, telling FOX News she "will bring votes that Obama can't."

"Clinton brings meat to the ticket," he said. "It's a dream ticket."

Click here to read what Women on the Web are saying about Sen. Clinton’s announcement.

Obama's campaign did not respond to Clinton floating the possibility of being his running mate, but he did praise her repeatedly.

"I can tell you that what gets Hillary Clinton up in the morning -- even in the face of tough odds -- is ... an unyielding desire to improve the lives of ordinary Americans, no matter how difficult the fight may be," he said.

Ahead of Tuesday's concluding primaries, Obama did try to set the stage for reconciliation, praising Clinton's endurance and determination and offering to meet with her - on her terms - "once the dust settles" from their race.

FOX News' Aaron Bruns, Major Garrett and Chad Pergram and The Associated Press contributed to this report.