WRAPUP 4-U.S. Democrats in dead heat ahead of big vote

(Adds Romney quotes)

By Ellen Wulfhorst

ST. LOUIS, Feb 3 (Reuters) - Democrats Hillary Clinton and Barack Obama were in a virtual dead heat two days before the biggest round of presidential voting so far while John McCain tried to nail down the Republican nomination for the White House.

With 24 states holding nominating contests from coast to coast on Tuesday, the candidates crisscrossed the country, leading rallies and urging supporters to get out and vote.

The Democratic race narrowed to almost a draw in national polls while McCain hoped to win enough delegates to the Republican convention to be the party's presidential nominee in the November presidential election.

Obama held a slight lead in California, the biggest prize of all where Clinton once led, and was virtually tied with her in New Jersey and Missouri -- three of the states voting on "Super Tuesday" -- in a Reuters/C-SPAN/Zogby poll released Sunday.

Obama picked up a high-profile endorsement in Los Angeles when Maria Shriver announced her support for him -- a few days after her husband, California Gov. Arnold Schwarzenegger, endorsed McCain. Shriver's uncle, Sen. Edward M. Kennedy, and cousin Caroline Kennedy, the daughter of former President John F. Kennedy, announced their support for Obama last week.

"I thought, if Barack Obama were a state, he'd be California," Shriver said. "... Diverse. Open. Smart. Independent. Bucks tradition. Innovative. Inspiring. Dreamer. Leader."

While Obama and Clinton vied to win the most delegates, they also were making the argument of being the most electable candidate to face McCain in November.