BY BETH REINHARD AND LESLEY CLARK

Foreshadowing what is likely to be one of the most hard-fought congressional races in South Florida in years, Democrat Raul Martinez has raised more than $615,000 in just two months to try to unseat Republican U.S. Rep. Lincoln Diaz-Balart.

Democratic party officials said the haul is the second-biggest fundraising start ever by a congressional challenger, though not all candidates have reported their donations for 2008.

The Martinez campaign said more than half of the contributions came from Republicans, though that claim could not be confirmed because the donors' names will not be made public until April 15.

Martinez served as mayor of Hialeah, one of the most Republican cities in Florida, for more than two decades.

''The mayor has an appeal that transcends party lines. He always has,'' said his campaign consultant, Jeff Garcia.

The race for Diaz-Balart's district, along with the neighboring Democratic challenges facing Cuban-American Republicans Mario Diaz-Balart and Ileana Ros-Lehtinen, is shaping up as a referendum on Miami-Dade politics.

Will the politically dominant Cuban-American community stick with the GOP and its hard line against the Cuban government, as they have for nearly three decades, or is their loyalty fading along with Fidel Castro's health?

''There is a real desire and appetite for change,'' Martinez said.

A spokesman for Lincoln Diaz-Balart, who has faced only token opposition since his election in 1992, said he raised more than $600,000 in the first three months of the year -- about the same amount Martinez raised since getting into the race in late January. Diaz-Balart's latest donations bring his campaign account to more than $1.4 million, according to the campaign.

A spokesman said the congressman had collected 7,900 voter signatures -- twice as many as required -- to qualify for the 2008 ballot instead of simply paying the filing fee of about $10,000. Martinez's campaign said he was planning to pay the fee.

''This election is not about collecting cash,'' said the Diaz-Balart campaign in a statement. ``This election is also about who can most effectively and honorably serve and represent the residents of the 21st Congressional District, and that's Lincoln Diaz-Balart.''

Diaz-Balart has received early endorsements from the Police Benevolent Association and the Firefighters Association.

The only challenger running for Congress who appears to have outraised Martinez is Texas businessman Michael Skelly, a Democrat who is said to have raised $750,000 in the past quarter.

Sarasota businessman Vern Buchanan, a Republican whose successful 2006 bid to replace U.S. Rep. Katherine Harris was one of the most expensive races in history, raised $588,096 in the first three months.

Democrat Christine Jennings, who is trying to avenge her narrow loss to Buchanan two years ago, is among the five or six Democrats the party is banking on to pick up at least one Florida seat and expand its majority in Congress.

A statement from Diaz-Balart slated to go out to supporters Thursday says ``the campaign will continue to raise all the funds necessary to get its message out to every voter. . . . Nonetheless, groups from outside our community are already talking about targeting our race with outside dollars and outside people. That's why your support is so critical, and why Lincoln feels so honored to have you on his team.''