McCain, the early front-runner whose campaign nearly unraveled six months ago, won in New York, New Jersey, Connecticut and Delaware to gain all 198 Republican delegates at stake there. He also put Illinois and Oklahoma in his column.
Former Arkansas Gov. Mike Huckabee won in Alabama and in his home state. He also triumphed at the Republican West Virginia convention.
Romney, the former governor of Massachusetts, won a home state victory. He also took Utah, where fellow Mormons supported his candidacy.
Democrats played out a historic struggle between Clinton, seeking to become the first female president and Obama, hoping to become the first black American to win the White House.
Clinton won at home in New York as well as in Massachusetts, New Jersey, Oklahoma, Tennessee and Arkansas, where she was first lady for more than a decade.
He marched through Georgia and won Alabama as well as Delaware, Utah and his home state of Illinois.
After an early series of low-delegate, single-state contests, Super Tuesday was anything but small - its primaries and caucuses were spread across nearly half the country in the most wide-open presidential campaign in memory.
The result was a double-barreled set of races, Obama and Clinton fighting for delegates as well as bragging rights in individual states, the Republicans doing the same.
Polling place interviews with voters suggested subtle shifts in the political landscape, potentially significant as the races push on through the campaign calendar.
McCain held a small edge among voters calling themselves Republicans, a group he had not won in any of the earlier primaries or caucuses. As usual, he was running strongly among independents.
Romney was getting the votes of about four in 10 people who described themselves as conservative. McCain was wining about one-third of that group, and Huckabee about one in five.
Overall, Clinton was winning only a slight edge among women and white voters, groups that she had won handily in earlier contests, according to preliminary results from interviews with voters in 16 states leaving polling places. Obama was collecting the overwhelming majority of votes cast by blacks.
Clinton was gaining the votes of roughly six in 10 Hispanics, and she hoped the edge would serve her well as the race turned west to Arizona, New Mexico and California, the biggest prize with 370 delegates.
The allocation of delegates lagged the vote count by hours. That was particularly true for the Democrats, who divided theirs roughly in proportion to the popular vote.
Nine of the Republican contests were winner take all, and that was where McCain piled up his lead.
Overall, he had 300 delegates to 93 for Romney and 61 for Huckabee. It takes 1,191 to clinch the presidential nomination at next summer's convention in St. Paul, Minn.
Overall, Clinton had 291 delegates to 243 for Obama, out of the 2,025 needed to secure victory at the party convention in Denver. Clinton's advantage is due to her lead among so-called superdelegates, members of Congress and other party leaders who are not selected in primaries and caucuses - and who are also free to change their minds.