BOCA RATON, Fla. — Using billboards, television advertisements and finely honed voter lists, Republicans here and in other battleground states have intensified an effort to lure a small but potentially significant group of new or wavering voters from President Obama.

Focused on South Florida, Ohio and Nevada, the Republican Jewish Coalition, backed mostly by the casino magnate Sheldon Adelson, a Zionist, has begun spending $6.5 million on an air-and-ground strategy to reach Jewish voters who may view Mr. Obama as unreliable on the question of Israel’s security. Jewish voters, who generally vote for Democrats in big numbers, overwhelmingly supported Mr. Obama in 2008, giving him 78 percent of their vote, according to exit polls conducted by Edison/Mitofsky.

In Florida, where the largest share of the $6.5 million is being spent, one of the group’s most visible messages is along Interstate 95 and the Florida Turnpike, including in Boca Raton and in Broward County, areas with large Jewish populations. A series of red-and-blue billboards lament: “Obama ... Oy Vey!!” Then ask, “Had enough?”

Last week, the coalition began the first in a series of “buyer’s remorse” television ads featuring a Jewish voter concerned about Israel and the economy who declared that he would not vote for Mr. Obama this time around.