Clinton's three triumphs ended a month of defeats for the former first lady, and she told jubilant supporters, "We're going on, we're going strong and we're going all the way."

Obama won the Vermont primary, and sought to counter Clinton's claims that the night had been a race-altering event. "We have nearly the same delegate lead as we did this morning and we are on our way to winning this nomination," he told supporters in Texas.

He also led in caucuses in Texas that began 15 minutes after the state's primary polls closed.

A final split of the 370 delegates at stake in the four states lagged far behind the vote count.

Both Democrats called McCain — a Senate colleague — to congratulate him on his triumph in the Republican race.

The 71-year-old Arizona senator surpassed the 1,191 delegates needed to win his party's nomination, completing a remarkable comeback that began in the snows of New Hampshire eight weeks ago. President Bush invited him to lunch — and an endorsement — at the White House on Wednesday. Read More