Changing demographics and the turbulent economy have created the first challenging races for a pair of Republican Cuban-American incumbents in South Florida.

For Rep. Mario Diaz-Balart (R-Fla.), the issue is demographics. His district has grown more Democratic and less Cuban over the years, and now includes more Hispanic voters of Colombian, Nicaraguan and other Latin American descent.

“The one, it’s the opponent,” said Susan McManus, a political science professor at the University of South Florida. “In the other, it’s the demographics.”

The focus on the economy is only making the environment more challenging for the two Diaz-Balart brothers.

“It’s hurting Republicans because, rightly or wrongly, [voters] associate the downturn with President Bush,” said pollster Dario Moreno, a professor at Florida International University and director of the Metropolitan Center. His polling shows 47 percent of Floridians now see the economy as their No. 1 issue.

That means the issue of Cuba could fade in importance, creating problems for both incumbents but especially for Lincoln Diaz-Balart. While both Diaz-Balart brothers are nationally known as hard-liners against the Castro regime, Lincoln Diaz-Balart, who was born in Havana, is even more associated with the issue than his younger brother, according to Moreno, himself a Cuban-American.

“When people are concerned about their jobs, they’re less worried about foreign policy,” Moreno said.

Polls show a pair of tight races. A recent survey by Telemundo found Lincoln Diaz-Balart ahead of Martinez 48 percent to 43, while Mario Diaz-Balart led Democrat Joe Garcia, a well-known political operative and former chairman of the Miami-Dade County Democratic Party, 43 percent to 41.

That’s big news, since the Diaz-Balart brothers have routinely carried their districts with ease against relatively light competition. In 2006, a tough year for Republicans nationally, eight-term incumbent Lincoln Diaz-Balart won 60 percent of the vote while his brother, who is in his third term, won 59 percent.

In Cuban-American candidates Martinez and Garcia, Democrats note they have found two challengers capable of serious fundraising.

While the Diaz-Balart brothers will have more cash on hand in the final weeks before the election, both challengers have enough money to keep things interesting. In addition, the Democratic Congressional Campaign Committee is already running an ad in both districts and has reserved additional airtime.

Republicans have taken note. The National Republican Congressional Committee has reserved $1 million in ad time in each district, and has already begun running an ad against Raul Martinez.

The Miami Herald recently endorsed Rep. Mario Diaz-Balart as well as Raul Martinez.