A look at the 'must win' states that could prove crucial in deciding the presidential nominees
CALIFORNIA
441 Democrats 173 Republicans
With one-fifth of the delegates in each race, California is the biggest prize for Republicans and Democrats alike, and all the leading candidates have campaigned in the state this week. Tracking polls show Clinton with a clear lead among California Democrats, and she has been cultivating Hispanic voters in the southern part of the state as well as working-class voters during a three-day visit.
Obama has spent less time in California but, although he does not expect to win the state, he should poll strongly in a state with more than 300 colleges and universities, and can expect to win a good proportion of the delegates.
McCain has moved into the lead among Republicans in recent weeks, and he will benefit from the support of governor Arnold Schwarzenegger, who commands a powerful political machine in the state. California's Republican primary is not winner-takes-all, however, so Romney can also expect to pick up some delegates.
ILLINOIS
185 Democrats70 Republicans
Obama is confident of a big victory in his home state, especially in Chicago, where he worked as a community organiser after leaving Harvard Law School, later becoming a state senator. Clinton was born in the Chicago suburbs, but she has few links to Illinois today, and the district in which she grew up is now solidly Republican.
Clinton is confident of bringing in a respectable haul of delegates from Illinois, however, particularly from the southern part of the state.
McCain is the clear frontrunner in the Republican primary, but the state awards its delegates proportionally, so Romney can expect to pick up some of them, as could Huckabee, who appeals to the state's sizeable community of evangelical Christians.
GEORGIA
103 Democrats 72 Republicans
Georgia is among the southern states where Obama hopes to reprise his spectacular triumph last week in South Carolina, where a massive turnout of African-American voters delivered him a two-to-one victory over Clinton. The latest poll gives Obama a six-point lead, not enough to make a big difference in delegate numbers; Clinton was campaigning in Atlanta this week in an effort to shore up her share of the vote. She still enjoys the support of many black leaders, although African-Americans appear to be moving towards Obama.
Huckabee is leading the Republican field in Georgia, much to McCain's satisfaction and Romney's annoyance. The greatest threat to a Huckabee win could come from conservative talk radio hosts who are singing a chorus of praise for Romney as the only candidate who can defeat McCain.
MASSACHUSETTS
121 Democrats 43 Republicans
Obama has the backing of Massachusetts governor, Deval Patrick, and the state's two US senators, Edward Kennedy and John Kerry, and it will be embarrassing for Kennedy if his chosen candidate fails to win the Bay State. Clinton remains ahead in most polls, however, and she enjoys the support of Boston mayor Thomas Menino, who presides over a powerful political machine. Kennedy is hugely popular but he has not faced a contested election since 1994, when Romney challenged him, so his organisation may be a little rusty.
Romney has a 23-point lead over McCain in the most recent Massachusetts poll, but the primary is a proportionate one, so, even in his home state, Romney is unlikely to win all the delegates. Both of the leading Boston newspapers, the liberal Boston Globeand the conservative Boston Herald, have endorsed McCain.
NEW JERSEY
127 Democrats 52 Republicans
Clinton is ahead in New Jersey, which often follows political trends in neighbouring New York, but Obama has been campaigning in the state, particularly in Newark, which has a large black population. Newark's mayor Cory Booker, another rising African-American politician, is a key Obama supporter, but his efforts may not be enough to tip the state away from Clinton.
On the Republican side, McCain has a double-digit lead over Romney, and Giuliani's support should help to secure the state for the Arizona senator. A judge ruled this week that New Jersey voters who cast an absentee ballot for a candidate who has withdrawn from the race can vote again on Tuesday for one of the remaining candidates.
NEW YORK
281 Democrats 101 Republicans
Clinton enjoys a 28-point lead in her home state, according to the latest Gallup poll, but Obama hopes to poll well in New York City, where the young, the well-heeled and African-Americans all form part of his natural constituency. Clinton hopes for a huge turnout of her supporters in rural, upstate New York, so that she can pick up extra delegates in districts where Obama polls poorly.
Giuliani's withdrawal from the Republican race has left New York wide open for McCain, who had already moved into the lead in the state more than a week ago. Giuliani's endorsement of McCain should ensure that Romney will not come close to winning the Empire State. New York has a winner-takes-all primary and its 101 delegates form a hefty chunk of the 1,191 needed to win the party's nomination.