WASHINGTON (CNN) -- In what he called a "defining moment for our nation," Sen. Barack Obama on Tuesday became the first African-American to head the ticket of a major political party.

Obama's steady stream of superdelegate endorsements, combined with the delegates he received from Tuesday's primaries, put him past the 2,118 threshold, CNN projects.

"Tonight we mark the end of one historic journey with the beginning of another -- a journey that will bring a new and better day to America," he said.

"Tonight, I can stand before you and say that I will be the Democratic nominee for president of the United States." Watch Obama say he'll be the nominee »

Obama's rally was at the Xcel Energy Center in St. Paul, Minnesota -- the same arena which will house the 2008 Republican National Convention in September.

Speaking in New York, Sen. Hillary Clinton, congratulated Obama for his campaign, but she did not concede the race nor discuss the possibility of running as vice president.

"This has been a long campaign, and I will be making no decisions tonight," she said. Watch Clinton congratulate Obama »

There were reports earlier in the day that she would concede, but her campaign said she was "absolutely not" prepared to do so.

Two New York lawmakers also told CNN on Tuesday that during a conference call Clinton expressed willingness to serve as Obama's running mate in November. Watch the latest on a possible joint ticket »'

One source told CNN that Clinton told those on the call that if asked by Obama, she would be interested in serving as his running mate.

One of the lawmakers said Clinton's husband, former President Bill Clinton, has been pushing the idea privately for several weeks.

The Clinton campaign maintains the New York senator merely said she would do whatever is in the party's best interest, and that her comments Tuesday are no different than what she has been saying for weeks.

Clinton said she would meet with supporters and party leaders in the coming days to determine her next steps. She also asked people to go to her Web site to "share your thoughts with me and help in any way that you can." Watch what could be in store in Clinton's future »

CNN has projected that Clinton will win the primary in South Dakota and Obama will take Montana. Those states marked the final contests in the primary season.

Obama praised Clinton's campaign. He has been speaking favorably of the New York senator as his focus has turned toward the general election and his battle against John McCain, the presumptive GOP presidential nominee.

"Sen. Hillary Clinton has made history in this campaign not just because she's a woman who has done what no woman has done before, but because she's a leader who inspires millions of Americans with her strength, her courage, and her commitment to the causes that brought us here tonight," he said.

Diving into general election mode, Obama turned his attacks to McCain, saying it's "time to turn the page on the policies of the past."

"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," he 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."

Earlier Tuesday night, McCain portrayed himself as the candidate of "right change."

"No matter who wins this election, the direction of this country is going to change dramatically. But the choice is between the right change and the wrong change, between going forward and going backward," he said in Kenner, Louisiana.

CNN's Candy Crowley, Jim Acosta, Suzanne Malveaux, Paul Steinhauser and Robert Yoon contributed to this report.

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