This could be a defining day in the 2008 race for the White House. Millions of Americans are heading to the polls to cast primary ballots on Super Tuesday.
The contests offer an enormous chunk of the delegates needed to win each party's nomination at the political conventions this summer.
Two-dozen contests from coast to coast are delivering 1,023 Republican and 1,681 Democratic delegates. The number needed to win the nomination: 1,191 Republican and 2,025 Democratic.
Party rules are stacked against a knockout for Democrats. All their primaries and caucuses award delegates proportionately, so coming in second counts.
In the Republican field, nine contests offer all delegates to the winner.