Democratic voters across the city turned out to vote and canvass for Senators Clinton and Obama today in a primary matchup — between a woman and a black man — that many say they are viewing as a historic moment for the party. New York State is one of the biggest prizes for both parties’ presidential hopefuls on a day in which 24 states will select nominees for the White House.

In Harlem and Morningside Heights this morning, volunteers for Mr. Obama manned street corners, passing out fliers to voters on their way to the polls.

The owner of a day care center in Harlem, Tiwana Robinson, said casting her vote for Mr. Obama felt more momentous than a usual day at the polls.

“This is the first time in history we’ve ever had a black runner go this far, so we can definitely see there is a change in the world right now,” Ms. Robinson, 34, said.

She added that she hopes to see the war in Iraq end and her taxes cut. “A lot of his issues, I’m feeling. He’s going for change and I think it’s time for change,” she said. “It’s getting crazy on the middle class.”

At 72nd street on the Upper West side, voters wore Obama and Clinton buttons and handed out campaign literature.

Anne Cunningham, 73, who distributed fliers for Mrs. Clinton, said that the former first lady’s run had drawn unfair criticism because of her gender. “I, as a woman, feel it’s frustrating the way she has been treated in this campaign because of her sex,” Ms. Cunningham said. “She’ll make a good president. She has a lot of experience, she’s personable, and she’s a lot stronger than people realize as far as combative strength and decision making.”

Across the five boroughs yesterday, New Yorkers added their voice to the battle that’s been brewing for more than a year between their home state senator and Mr. Obama.

In interviews at polling places in Manhattan and Brooklyn, voters cited many of the same concerns expressed by those who preceded them to the polls in Iowa, New Hampshire, South Carolina, and the other early primary states. Several Obama supporters said they were won over by the Illinois senator’s calls for change, while other voters said they were swayed — for better or worse — by Mrs. Clinton’s eight years in the White House as first lady.

“I voted for him because I really saw that he has a fresh, different look for the direction the country needs to be going in,” an Upper West Side resident who works as a research scientist at the Frick Museum, Rodica Krauss, said of her support for Mr. Obama. Ms. Krauss, 57, said she had switched her registration from Republican to vote for him. Mrs. Clinton, she said, has “been an insider for a long time. Somehow she will follow the same direction.”

David Motson, a librarian at Columbia University, said he voted for Mrs. Clinton, citing her experience.

“She will have less of a learning curve,” Mr. Motson, 44, said after casting his vote at the Margaret Douglas School on West 122nd Street. “She had to get something out of living in the White House and being married to a president for eight years.”

A Brooklyn Heights resident, Gery Diamond, said she switched her support to Mr. Obama only in the last two weeks. “I wanted to vote for Hillary,” she said. “I liked her before her campaign tactics in South Carolina when she said false things about Obama. I was let down by the Clintons and how he campaigned with such vitriol. It tilted me.”

A circulation supervisor for a magazine in Manhattan, Giovanni Vitacolonna, 59, said he had supported John Edwards before he dropped out of the race last week.

“I voted for Obama because I don’t want to do Billary again,” he said, referring to President Clinton’s influence in his wife’s campaign. “If she did it on her own I might be a little more comfortable.”

On the Republican side, Senator McCain of Arizona entered as the heavy favorite both nationally and in New York, where he has drawn the support of most party leaders.

He began his Super Tuesday bright and early today, predicting victory before a crowd of supporters at Rockefeller Center.

“We’re going to win today, and we’re going to win the nomination, and we’re going to win the presidency,” the Republican front-runner said, standing alongside his former rival, Mayor Giuliani, and a host of other state supporters at 7:30 this morning.

Mr. McCain spoke for about 10 minutes before heading across the country for a get-out-the-vote rally in San Diego this afternoon. He plans to watch the returns from his home state of Arizona tonight.

With leads in a majority of the 21 states holding Republican primaries or caucuses today, Mr. McCain could wake up tomorrow on the cusp of clinching the party’s nomination. Helped by Mr. Giuliani’s withdrawal and endorsement last week, he holds a wide lead in New York over Mitt Romney.

Mr. Giuliani, a one-time front-runner, had hoped to be in Mr. McCain’s place today. But he appeared in good spirits, taking the stage and grabbing an “Irish for McCain” sign from a supporter. “Welcome to the St. Patrick’s Day Parade!” Mr. Giuliani quipped, drawing laughs from the crowd. “Irish for McCain, all New Yorkers for McCain!”

Mr. Giuliani acknowledged the other major city event drawing the attention of New Yorkers today — a Super Bowl victory parade — urging the crowd to work in a trip to the polls around their support for the New York Giants. “I know you’re all going to go to the Giants parade, right? Well, before you go, or after it’s over, I want you to vote for John McCain for president of the United States.”

While Mr. McCain led in New York, a poll released this morning suggested that Mr. Romney may have gained an edge in California, which could be his last hope at preserving a viable campaign. The Reuters/C-Span/Zogby poll showed the former Massachusetts governor with a seven-point lead, 40% to 33%. A SurveyUSA poll released today, however, showed a dead heat, with Mr. McCain holding a one-point lead, 39% to 38%.

Among Democrats, Mrs. Clinton has long been heavily favored to win her home state, but the Obama campaign refused to cede New York entirely and tried in particular to make inroads in the city. Democratic party rules allocate delegates proportionally by congressional district, meaning that even if Mr. Obama was unlikely to win the popular vote statewide, he could still hope to prevent Mrs. Clinton from sweeping New York’s triple-digit delegate trove.

A top New York fund-raiser for Mr. Obama, Gordon Davis, sent an e-mail to potential donors on Friday seeking last-minute contributions to fund Election Day activities. The e-mail said the campaign had targeted five districts in the city where they hoped to “steal a lot of delegates from Hillary.” Those districts comprised much of Manhattan, including Harlem, and parts of Brooklyn.

The Obama campaign was out in force in many of those areas, and in some cases with more visibility than the Clinton campaign.

On a day when a presidential election and a Super Bowl parade competed for attention in the city, some New Yorkers turned to multi-tasking. On a downtown A train yesterday morning, four teenagers decked out in Giants jerseys and face paint traversed a subway car, asking riders if they had voted and urging them to pull the lever for Mr. Obama.