WASHINGTON — Senator Obama will be making a play to shore up Jewish support in the swing state of Florida today even as Republicans launch a campaign there aimed at showing he is weak on Israel.

The Republican Jewish Coalition purchased newspaper ads in three southern Florida cities for the same day the likely Democratic presidential nominee will address the B’nai Torah Congregation in Boca Raton, Fla.

The ad poses three questions to the senator that touch on his call for a “summit of Muslim nations, including Iran and Syria”; the involvement of one of his senior military advisers, Merrill McPeak, who criticized the pro-Israel lobby as a cause for the lack of progress toward a peace deal between Israel and the Palestinian Arabs, and the fact that a foundation Mr. Obama worked for in the 1990s funded a Chicago group known as the Arab American Action Network.

A spokesman for the Obama campaign yesterday said the candidate was a strong friend of Israel. “Senator Obama is a strong friend of the American Jewish community and Israel, and will make ensuring Israel’s security a high priority of his presidency,” Tommy Vietor said. “Desperate partisan attacks like this one are designed to distract voters from the fact that John McCain offers nothing more than a third term of George Bush’s foreign policy.”

In the last month, Mr. Obama has stepped up his campaign to win the votes of Jewish leaders. In campaigning against Senator Clinton in Pennsylvania, Mr. Obama met with Philadelphia Jewish leaders and said he was influenced by Jewish writers such as Saul Bellow and Philip Roth. More recently, Mr. Obama has rejected suggestions that he would order diplomacy with Hamas, the Palestinian group that seized control of Gaza last June and is considered a terrorist entity. Mr. Obama has also granted interviews with a New York Times columnist, David Brooks, and an Atlantic Monthly reporter, Jeffrey Goldberg.

Nonetheless, some polling suggests Mr. Obama lags behind Senator Kerry’s support among Jewish voters in the 2004 presidential campaign. The majority of American Jewish voters are Democrats. The latest Gallup poll found that Mr. Obama secured only 61% of American Jews.

“If Obama loses 20 points off of Kerry from the Florida Jewish population, he gets 57% nationwide, and in Florida it’s roughly 170,000 votes based on my estimates. Those are serious numbers,” the executive director of the National Jewish Democratic Council, Ira Forman, said. “But relatively few elections are decided by less than 170,000 votes, but sometimes they are, most notably Florida, which was decided by 500-some votes in 2000.”

Mr. Forman added that he expected the Obama campaign would make up the difference by November. “How much of this is a pro-Israel vote is the question,” he said. “My guess is that this is a whole lot less. This is because McCain is currently perceived by some in our community as a moderate, but by the time this campaign is over most of that vote will come back to Obama.”

Mr. Forman called the current Republican attacks on Mr. Obama’s national security posture the equivalent of “throwing the kitchen sink at him.” “From an objective perspective, Obama’s position on Israel — his voting record and his statements are extremely defensible,” he said.

The national security adviser to the McCain campaign, Randy Scheunemann, yesterday said he was confident that “when American voters compare senator McCain’s staunch support of Israel with Senator Obama’s record, including his desire to meet unconditionally with an Iranian leader who has called Israel a stinking corpse, they will conclude Senator McCain has the experience and judgment to secure American interests in the Middle East.”

During a Democratic debate last July, Mr. Obama said he would meet without conditions with the leaders of Cuba, Iran, North Korea, Syria, and Venezuela. Since then, his campaign advisers have softened the position, stating that the desire or courage to meet with adversaries is not to be confused with a plan to immediately meet with them.