By MICHAEL LUO

MIAMI — Senator John McCain took his sharp criticism of Senator Barack Obama on foreign policy to a new venue on Tuesday, using a speech on Cuba here in a critical swing state to again argue that Mr. Obama is too willing to talk to America’s enemies.

The question of how to engage controversial foreign foes has emerged as perhaps the central dispute between Mr. McCain, the presumptive Republican nominee, and Mr. Obama, his likely Democratic opponent.

Mr. McCain accused Mr. Obama on Tuesday of shifting his position on normalization of relations with Cuba, saying he voiced support for it previously but was now offering caveats. In doing so, Mr. McCain homed in anew on Mr. Obama’s stated willingness to conduct a dialogue with controversial foreign leaders.

“Now Senator Obama has shifted positions and says he only favors easing the embargo, not lifting,” Mr. McCain said. “He also wants to sit down unconditionally for a presidential meeting with Raúl Castro. These steps would send the worst possible signal to Cuba’s dictators — there is no need to undertake fundamental reforms, they can simply wait for a unilateral change in U.S. policy.”

Mr. McCain’s words, in keeping with his continuing efforts to paint Mr. Obama as inexperienced and reckless on national security, played well before an important constituency in South Florida, an audience full of Cuban exiles, many of whom had suffered under Fidel Castro’s government.

The Obama campaign quickly sent out a response from Senator Christopher J. Dodd of Connecticut and Gov. Bill Richardson of New Mexico, both supporters with foreign policy experience, who argued the country needed a change from a Bush administration approach that had for much of the past seven years frowned upon dialogue with certain adversaries, though it has more recently shown signs of shifting.

“John McCain needs to explain why continuing to do exactly what George Bush has done will somehow produce a different result,” Mr. Dodd said.

Mr. Obama’s record on Cuba is mixed. While campaigning for the Democratic presidential nomination, Mr. Obama has said he favors taking steps to normalize relations only if the Cuban government takes meaningful steps toward democracy.

When he was running for United States Senate in Illinois in 2003, however, Mr. Obama said in a questionnaire that he favored normalizing relations with Cuba but did not offer any qualifiers.

Mr. McCain has his own conflicting past statements to contend with. When he was running for president in 2000, Mr. McCain stood out for supporting normalizing relations with Cuba, even if Fidel Castro remained in power, provided the government went through certain steps to democratize its country, similar to the road map to normalizing relations with Vietnam.

The difference with his position now appears to be primarily one of emphasis, with Mr. McCain forcefully contending the Cuban embargo should be kept in place until certain specific political freedoms are restored.

But Mr. McCain said in 1999, according to The Miami Herald, that if Mr. Castro took certain steps, he would be willing to wait until the goal of supervised free elections before beginning steps toward normalization.

Both campaigns have engaged in sound bites and simplifications of these major policy differences, but in explaining, defending and attacking each other, they have displayed clear distinctions in their beliefs in the efficacy of diplomacy.

The jumping-off point for the debate has continued to be Mr. Obama’s response at a Democratic debate last year in which he said he would be willing to sit down without preconditions with the leaders of certain foes of the United States, including Cuba and Iran. The response exposed him to repeated broadsides, at first from Senator Hillary Rodham Clinton, his Democratic opponent, and now Mr. McCain.

On Iran, Mr. Obama has since sought to draw a distinction between such engagement by an Obama administration and his personal involvement as president, saying his involvement would require the Iranians’ meeting certain benchmarks.

Mr. Obama has repeatedly argued that the United States needs to be willing to engage in dialogue with all countries, including enemies, while Mr. McCain has stuck much closer to the stance that largely characterized the Bush administration, maintaining that diplomatic engagement is a privilege.

More recently, however, the Bush administration has edged toward a greater willingness to engage countries like Syria, Iran and North Korea, where the administration has claimed important progress in reaching an agreement to dismantle the country’s nuclear program.

Mr. McCain has said he does not oppose lower-level discussions with adversaries under certain conditions but singled out the notion of a presidential-level visit as a carrot that must be handled with care so as not to enhance the prestige of a despotic leader.

Expanding on his critique of Mr. Obama while speaking with reporters aboard his campaign bus on Tuesday, Mr. McCain repeatedly voiced skepticism at what he described as the “school of thought” that urges, “Let’s sit down and talk, and we can work everything out,” arguing such efforts had repeatedly failed in history.

“It’s happened time after time after time,” he said. “Unfortunately, generally speaking, unless there are reasons and national security interests involved in both parties, then they don’t work.”