CHICAGO — Sen. Barack Obama shattered racial barriers and ushered in a new era of American politics on Tuesday by decisively defeating his Republican rival to become the 44th president of the United States of America.

Obama, the 47-year-old son of a Kansan mother and a Kenyan father, topped Sen. John McCain of Arizona to conclude the longest, most expensive campaign in the country’s history. The sentiment in Chicago’s Grant Park, where Obama declared victory, was clear: America had gotten its money’s worth.

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To become the nation’s first black president-elect, the Illinois senator banded together a coalition augmented by minorities and young people who embraced his message of change and repudiated the past eight years of Republican rule under President George W. Bush ’68.

“This victory alone is not the change we seek,” Obama said in his acceptance speech here. “It is only the chance for us to make that change.”

Vote-counting continued in the early hours of Wednesday morning, but with more than three-quarters of precincts reporting as of 4:30 a.m., Obama had amassed 52 percent of the popular vote to McCain’s 47 percent. In the electoral college, CNN projected an Obama lead of 338 to 163, with votes in Missouri, Indiana and North Carolina yet to be assigned.

A redefined electoral map lifted Obama above the 270 electoral votes required to win. Obama won every state Sen. John Kerry ’66 picked up in 2004 while also turning states formerly loyal to President George W. Bush ’68, such as Florida and Virginia.

Though the campaign was cautious about declaring victory too early Tuesday, the 240,000 supporters massing in Chicago’s Grant Park began celebrating long before the president-elect appeared.

The mood at McCain’s Election Day headquarters in Phoenix became subdued after results began filtering in from around the country and hinted that the Republican was likely headed for defeat. Shortly after the television networks called the election for his opponent, McCain appeared before his supporters and praised the Democrat for “inspiring the hopes of many millions of Americans.”

“I urge all Americans who supported me to join in not just congratulating him,” McCain said, “but in offering our next president our goodwill and earnest effort to find ways to come together.”

Obama’s victory in the presidential contest coincided with a series of Democratic victories in Congressional races, marking a potent political shift and rejection of eight years of Republican rule. At press time, toss-up Senate races in Oregon, Minnesota, Alaska and Georgia left a small chance that Democrats could attain a filibuster-proof 60-seat majority in the Senate.

Unlike during the fiercely contested Democratic primary in Connecticut, Yale students involved in campaigns largely ignored the state in the months leading up to the election. Instead they focused on battleground states such as Ohio, Pennsylvania and New Hampshire. Yale students volunteered for Obama in droves up until Election Day, with some skipping class to canvass neighborhoods in New Haven and across the country.

Connecticut was one of the first states to fall into Obama’s column Tuesday night, swinging 60 percent to 38 percent in the Democrat’s favor. Connecticut state officials predicted that turnout, driven by new voter registrations, would reach 90 percent in the state.

Obama’s platform of change resonated with youth, liberal and minority voters nationwide; they turned out at record rates Tuesday, causing long lines at polling stations across the country.

“If there is anyone out there who still doubts that America is a place where all things are possible, who still wonders if the dream of our founders is alive in our time, who still questions the power of our democracy,” Obama told his supporters, “tonight is your answer.”

The historic night started well for Obama, when networks called Pennsylvania for the Illinois senator. McCain had campaigned heavily in the state last week, hoping to make the Keystone State the turning point for an upset electoral victory. Losing the state left the Arizona senator with few options to reach the necessary 270 electoral votes.

In a pattern that spelled defeat for the Republicans, McCain repeatedly missed 2004 voting benchmarks in rural, dependably Republican areas, while Obama outpaced the vote totals of Kerry in many places.

The knockout blow came in Ohio, where Obama clipped McCain by four points, 51 percent to 47 percent. Strong turnout and big margins for the Illinois senator in Cleveland, Columbus and Dayton dealt a death blow to McCain. On the ground McCain’s field organization was simply outmatched, with Obama enjoying nearly a 2:1 advantage in field offices in the state.

Obama’s election completes one of the most meteoric rises in modern American politics. A mere state senator only five years ago, Obama stood triumphant Tuesday night in the city that handed him crushing defeat in a 2000 House race and yet lifted him to national prominence in his 2004 campaign for the U.S. Senate. It was here — on the South Side of Chicago — where Obama learned the possibility of hope to make change.

In a speech that repeatedly referenced Abraham Lincoln, Obama cast the election as less about his candidacy than about a resurgence of the ethos of public service. The nation faces great challenges, Obama said, and will require a “spirit of service, a new spirit of sacrifice” to navigate them.

“So let us summon a new spirit of patriotism, of responsibility,” he told the crowd in Grant Park, “where each of us resolves to pitch in and work harder and look after not only ourselves but each other.”

It was an emotionally fraught moment. Even Obama appeared to be holding back tears. The assembled masses swung between solemn attention and uproarious commotion; people cried and danced, sang and cheered. Elis lucky enough to be in Grant Park spoke highly of the experience. Chicago resident Brian Levin ’11 was among the crowd and watched just feet from Obama’s lectern.

“People of every imaginable background — people who had been denied the vote on the basis of race, and people who were involved in a campaign for the first time — were standing together to celebrate the dawn of this new age in American politics,” Levin said.

It took a few minutes to dawn on Rustin Fakheri ’12, who was also in Grant Park, that Obama had actually won. When it did, he said the feeling was “surreal.”

Peter Croughan ’12 said he would never forget witnessing this moment in history and all the enthusiasm and passion that it evoked.

“People were more fired up for a politician than they were for the Red Hot Chili Peppers,” he said.

It is difficult to predict how a President Obama will influence America’s black community, African-American Studies professor Jonathan Holloway said late Tuesday. He called Obama’s victory a “fascinating moment in history” but maintained that the election of a black president does not erase a centuries-old legacy of discrimination.

“You can’t change the history of the struggles that defined the African-American experience,” Holloway said by phone. “There is still work to be done. Racism is still alive and well.”

With the end of the longest campaign on record, Obama said he would now turn his attention to governing and the challenges facing the nation.

“The road ahead will be long,” he said. “Our climb will be steep. We may not get there in one year or even in one term. But, America, I have never been more hopeful than I am tonight that we will get there. I promise you, we as a people will get there.”

Paul Needham contributed reporting from Phoenix.