A total of 1,678 pledged delegates were at stake in the 22 state contests on Tuesday, with 2,025 delegates needed to win the nomination.

Among Democrats voting on Tuesday, a majority said that they were most concerned about the economy, outpacing those worried about the Iraq war or healthcare. Nine out of 10 Democratic voters said the economy was in bad shape.

A majority of Democrats in most states said they believed that Clinton was best suited to be commander in chief, while Obama had a similar edge among Democrats regarding who was more likely to unite the country.

For months now, the Obama and Clinton campaigns have viewed Tuesday as a decisive moment in the presidential race. When Clinton lost the first nominating contest, in Iowa, she noted that the 45 delegates at stake there were a mere fraction of the delegates at stake in the state contests on Tuesday.

Obama and his aides made similar remarks after his losses in New Hampshire and Nevada, and both he and Clinton increasingly spoke of the nomination fight as a two-way battle for delegates, pure and simple.

Clinton underscored this viewpoint by campaigning in California and Arizona, two states that voted on Tuesday, in the week before the South Carolina primary — signaling, in effect, that her strategy was much more focused on winning contests on Tuesday than on South Carolina.

Over the last week, however, public and private opinion polls have showed tightening races in states where Clinton had held substantial leads, including Massachusetts and New Jersey and California.