BY AMY DRISCOLL AND LESLEY CLARK

Miami-area Democrats, seeking to show they have national support for three key congressional races against Republicans, brought in some extra fire power Monday: U.S. Rep. Chris Van Hollen, head of the committee that helps House Democrats get elected.

Van Hollen of Maryland pledged that the three Democratic challengers will have the backing of the Democratic Congressional Campaign Committee he oversees.

''We are absolutely committed to winning these seats because we think there is a tremendous opportunity in South Florida,'' Van Hollen said, addressing the group at a Coral Gables restaurant.

Former Hialeah Mayor Raul Martinez is challenging Rep. Lincoln Diaz-Balart; former Miami-Dade Democratic Party Chairman Joe Garcia is running against Rep. Mario Diaz-Balart; and businesswoman Annettte Taddeo faces Rep. Ileana Ros-Lehtinen.

Republicans have focused so heavily on Cuba and Fidel Castro, Van Hollen said, that they've nearly excluded other ''pocketbook issues,'' including children's healthcare and energy policy that are increasingly preoccupying South Floridians.

''This is a one-note song that has been sung,'' Van Hollen said of the Castro issue.

The incumbents, who outlined a plan at a press conference Monday to cut taxes on gasoline, have rejected such criticism, pointing to various projects they have steered to South Florida, including restoring Social Security benefits for legal immigrants and money for local transportation projects and for dredging the Miami River.

But Democrats have signaled the ''one-note song'' is likely to be one of their lines of attack.

Van Hollen also noted that voter registration in the three districts has been shifting toward Democrats and independents, although Republicans still have the edge.

The three Democrats are ''on track,'' Von Hollen said, to qualify for extra help and funding from a Democratic program called Blue to Red, which assists qualified Democrats trying to win Republican congressional seats.

Martinez and Garcia also addressed the luncheon, organized by the Miami chapter of the Democratic Professionals Council, questioning how much the incumbents have helped the folks back home.

''The mantra here is, watch what they do, not what they say,'' Van Hollen said.

National Republican Congressional Committee Chairman Rep. Tom Cole dismissed the contention that the three Republicans are vulnerable, saying Van Hollen and the Democrats are welcome to "waste their resources down there.''

''You're not going to beat Ileana; you're not going to beat Mario; and you're not going to beat Lincoln,'' the Oklahoma Republican said at a briefing for reporters at NRCC headquarters in Washington.

"Those are candidates that are super well-prepared. They're active and aggressive both here and at home. They've got sizable war chests. . . . You're not going to catch them sleeping.''

Cole said he believes Republicans in many districts will be aided by ''prolonged exposure'' to the Democrats' protracted battle for the presidential nomination.

''Democrats have attractive candidates. . . . But I don't think they're going to win in those kind of seats in November,'' he said of the three South Florida seats. "I think all three [incumbents] will come back, and I think all three will come back comfortably.''