Commercials promoting Barack Obama's campaign for US president are in heavy rotation in California on Univision, the Spanish-language television channel that rivals the main US networks for viewer share in the southern part of the state.

"We're spending more money on Latino TV and radio than has ever been spent in a presidential primary," says Temo Figueroa, national field director with the Obama campaign. "Latinos have never had this amount of attention in a primary or played this important a role before."

Hispanics represent almost 20 per cent of California's electorate and reliably turn out to support Democratic candidates. Mr Obama and his rival Hillary Clinton are hoping to attract support from these voters on Tuesday when 22 states hold primaries. With 370 Democratic delegates up for grabs - more than a fifth of Tuesday's total - California is the biggest prize.

"It only takes three per cent of Hispanics to switch their votes to make a one point difference in [Californian] election returns," says Harry Pachon, president of the Tomás Rivera Policy Institute at the University of Southern California.

California is also a big fund---raising target. One in every five dollars raised for the Democratic frontrunners up to the third quarter of last year came from the state.

The Clinton campaign has been buying airtime and both camps are sending high-profile people out to stump for their cause. Antonio Villaraigosa, the mayor of Los Angeles and California's most prominent Latino politician, is campaigning on Mrs Clinton's behalf, as is Dolores Huerta, the co-founder of the powerful United Fruit Workers union.

Mr Obama has drawn support from María Elena Durazo, secretary treasurer of the Los Angeles County Federation of Labor, and Federico Peña, the Denver mayor and former cabinet member during Bill Clinton's administration.

Hilda Solis, a congresswoman who represents a heavily Latino district in east Los Angeles and is campaigning for Mrs Clinton, says when California holds its primary next week "Hispanics will determine who the next president will be".

Mrs Clinton has the broadest base of support among Latino voters in California, according to Kenneth Burt, political director for the California Federation of Teachers and the author of The Search for a Civic Voice: California Latino Politics . She is preferred by 60 per cent of likely Latino voters against 20 per cent for Mr Obama, according to a poll last week.

"But Ted Kennedy's endorsement of Obama [last week] could change that," says Mr Burt. "He has a Rolodex [of contacts] . . . he just has to activate it."

The association with the Kennedy family, which has deep ties with the Latino community, could become a turning point for Mr Obama in California. Robert Kennedy became a hero to Hispanics in the mid-1960s when, as a senator, he publicly supported striking Latino farm workers. He then won the California Democratic primary in 1968, thanks, in part, to the Latino vote.

The race for the Hispanic vote has highlighted the ill-feeling that simmered between Mrs Clinton and Mr Obama in other states.

Without mentioning Mr Obama's name, Ms Solis says "the other candidate is having difficulty attracting people to man phones who are bilingual".

But the Obama campaign has been most outraged by comments from Sergio Bendixen, Mrs Clinton's chief Hispanic pollster. "The Hispanic voter - and I want to say this very carefully - has not shown a lot of willingness or affinity to support black candidates," he told the New Yorker in a recent interview.

"That argument is not only divisive, it's wrong," says Mr Figueroa, pointing to a string of black officials - including Tom Bradley, a former Los Angeles mayor - who were supported by Latino voters. Mr Bendixen's comments are "a bold faced-lie", he adds.

Race aside, a key issue Hispanic voters are concerned about is the slowing US economy. One reason is that mortgage foreclosure rates are spiralling in Hispanic areas, such as Riverside and San Bernadino counties.

"People have been miserable for the last eight years and Hispanics have been at the bottom rung for everything," says Ms Solis.

Mr Burt says that Mrs Clinton could benefit from her husband's economic record. "Bill Clinton worked hard to win over the Hispanic community in his two terms as president," he says. "There's an identification that he brought good economic times."

Antonio Gonzalez, president of the non-partisan Southwest Voter Registration Education Project, agrees. "She's on the right side of the issues that Latinos care about," he says. The California primary is "Hillary's to lose".