Every day in this space during the Democratic and Republican conventions, Roll Call will publish poll results from a competitive Congressional district.

Rep. Lincoln Diaz-Balart, one of three Cuban-American Republican incumbents who Democrats are targeting in South Florida this year, is in the fight of his political life, according to a new poll conducted exclusively for Roll Call.

In a head-to-head matchup with Raul Martinez (D), the colorful and controversial former mayor of Hialeah, Diaz-Balart received 46 percent while the Democrat garnered 48 percent.

The poll of 632 likely voters was taken Aug. 24-26 for Roll Call by SurveyUSA, an automated opinion research firm. Voters were given the option of responding to the poll in either English or Spanish; the poll had a 4-point margin of error.

Diaz-Balart’s Miami-area district has been a Republican stronghold for years. Since running unopposed when he first won the seat in 1992 — and in four subsequent races — Diaz-Balart has skated to re-election, never winning less than 59 percent of the vote. What’s more, 21st district voters have strongly supported President Bush, giving him 57 percent in 2004 and 58 percent four years earlier.

But the Roll Call poll suggests that the Democrats’ hunch that the South Florida electorate is changing might be true. Not only was Diaz-Balart locked in a tight race with Martinez, but Sen. Barack Obama (D-Ill.) and Sen. John McCain (R-Ariz.) were tied in the poll, with each getting 48 percent of the vote.

Democrats’ optimism in Florida has been fueled by their belief that younger Hispanics who are not of Cuban descent are increasingly identifying with the Democratic Party.

Indeed, Cuban-Americans represented the backbone of the incumbent’s support in the Roll Call poll. Diaz-Balart, who was born in Havana and is the son and grandson of Cuban politicians, led Martinez in the Cuban community, 70 percent to 28 percent. Non-Cuban Hispanics preferred Martinez, 56 percent to 37 percent.

Younger voters are less reliable than their older counterparts when it comes to showing up at the polls, and many of the district’s younger voters are non-Hispanic Cubans. Voters who are 18 to 49 years old favored Martinez 55 percent to 38 percent, while voters older than 50 favored Diaz-Balart, 54 percent to 41 percent. Similarly, Obama led McCain among younger voters while McCain was the favorite among older voters.

In the Congressional race, Martinez also led among the district’s black and white voters.

Martinez, who was also born in Cuba, is a well-known figure in the district, having served as mayor of Hialeah for 24 years before retiring from the post in 2005. Martinez faced federal corruption charges in the 1990s; he was first convicted but appealed his conviction. The jury during his second trial deadlocked; in a third trial, he was acquitted of some charges and faced a hung jury on others.

Not surprisingly, Martinez’s favorable to unfavorable ratings are about even, with 33 percent of poll respondents saying they viewed him favorably and 32 percent saying they viewed him unfavorably — making his standing in the polls all the more noteworthy. Diaz-Balart had a 42 percent favorable rating and a 30 percent unfavorable rating.

The 21st district is just one of three in South Florida that national Democrats are targeting heavily this year. They are also high on businesswoman Annette Taddeo (D), who is challenging Rep. Ileana Ros-Lehtinen (R), and Dade County Democratic Chairman Joe Garcia, who is running against Rep. Mario Diaz-Balart (R), Lincoln Diaz-Balart’s brother.

Martinez and Garcia have been making the rounds in Denver during the Democratic National Convention this week.