Voters in 16 states across the US could decide today who will be the Democratic and Republican candidates in November's presidential election.
Vice-President Al Gore will almost certainly deliver a knockout blow to his opponent, former New Jersey senator Mr Bill Bradley. But the Republican frontrunner, Governor George W. Bush of Texas, may not get enough support to shake off the challenge of Senator John McCain of Arizona when the results come in from the big states of New York, California and Ohio.
The primary elections on what is called "Super Tuesday" will decide about one-third of the delegates to the nominating conventions of both parties next August. But that also means about 60 per cent of the required majority, so Mr Gore could end up tomorrow as a certain nominee for the Democratic Party.
For Mr McCain, the battle for New York with its 101 Republican delegates is crucial and the latest polls indicate a very close race. The McCain camp has privately conceded the California primary among Republicans, with its big prize of 162 delegates, but still hopes that the senator can win the parallel "beauty contest" for who is the most popular among all the candidates in both parties.
If Mr McCain wins this contest he will argue that he is more "electable" than Mr Bush in November when up against Mr Gore.
Mr McCain is also hoping to beat Mr Bush in the five New England states which vote today. A clean sweep there would win him 102 delegates, or one more than the number at stake in New York.
The race in New York has Mr McCain as an outsider against the Republican Party establishment, including the governor, Mr George Pataki, and the mayor, Mr Rudy Giuliani, which has swung solidly behind Mr Bush. Congressman Peter King is one of the few Republican politicians to support Mr McCain. All five New York newspapers have endorsed Mr McCain, which is unprecedented.
Unlike the New England states, where independents can vote in the Republican primaries, only registered Republicans can vote in New York and this makes Mr McCain's job harder. All his primary wins so far have been due to strong support from independents and even Democrats who have crossed party lines.
Mr Bush now has a total of 208 delegates, compared to 104 for Mr McCain and five for the third Republican candidate, a former ambassador, Mr Alan Keyes.
Mr McCain is complaining strongly about Bush campaign advertisements that he voted in the Senate against breast cancer research funding for a New York hospital. Mr McCain, who has a sister who suffered from breast cancer, insists he opposed funding going to the pet projects of local politicians instead of to the main medical research centre in Washington, but the ads seem to have damaged him in New York.
Mr Bradley is heading for a large defeat in New York, where he was once a basketball star and where he was the favourite two months ago against Mr Gore. Mr Bradley is also far behind in the polls in all the states voting today and he is being urged to retire after the results come in and leave a clear field to the vice-president.
Other states which are voting in Super Tuesday, in addition to California, New York, Ohio and the five New England states, are: Georgia, Hawaii, Idaho, Maryland, Minnesota, Missouri, North Dakota and Washington.