Critics in the foreign-policy establishment and from rival presidential camps said his idea could undercut pro-Western forces and legitimize leaders whose power the U.S. wants to undermine, including Iranian President Mahmoud Ahmadinejad. Increasingly, they are presenting his ideas as a radical departure from standard U.S. doctrine.

Presidents for decades avoided meeting directly with the leaders of Iran, Cuba and North Korea.

“If you look beyond Iraq, the entire diplomatic approach [of Senator Obama] seems to be kind of New Age: let’s talk to our enemies rather then reinvigorating our allies,” said Randy Scheunemann, the McCain campaign’s director of foreign policy. “It’s naive.”

The Obama campaign said it may be necessary to balance the Bush administration and the way it isolated hostile countries and alienated allies.

“I don’t think [what Obama’s proposing] is that much of a difference from what U.S. policy used to be.” said Anthony Lake, a senior foreign-policy adviser to Sen. Obama and a national-security adviser to President BillClinton. “It’s just different from what the other candidates are saying.”

Sen. Obama has sought to cast his candidacy as a rebuke of the hawkish foreign-policy line he sees as having led to the Iraq invasion and the diplomatic stalemates undermining U.S. efforts to end Iran’s and North Korea’s nuclear programs.

In speeches, Sen. Obama has said Washington’s global standing has plummeted in the past eight years, in part because of President Bush’s unwillingness to directly engage leaders such as Mr. Ahmadinejad or North Korea’s Kim Jong Il. Sen. Obama has said he would be willing to directly hold talks with these leaders during his first year to underpin efforts to stabilize the Middle East and Northeast Asia, provided proper preparations were made.

European and Arab diplomats have received the message warmly, fearing in particular a military confrontation with Iran.

“The notion that somehow not talking to countries is punishment to them…is ridiculous,” Sen. Obama said in a debate last year. “One of the first things that I would do in terms of moving a diplomatic effort in the region forward is to send a signal that we need to talk to Iran and Syria.”

Sen. Obama also has said he would be willing to reach out to Venezuelan President Hugo Chávez and Cuba’s new leader, Raúl Castro.

U.S. foreign-policy doctrine has traditionally held that American presidents should be discerning in where and when to use the prestige of their office. During U.S. engagements with the Soviet Union and China, summits by presidents Reagan and Nixon were withheld to the end of the diplomatic process rather than occurring at the start. U.S. leaders have been averse to photo opportunities with the likes of Cuba’s Fidel Castro, fearing it could provide them a propaganda tool.

Sen. Clinton has attacked Sen. Obama’s position on holding unconditional talks with Iran and Syria. She has generally voiced support for dialogue with such adversaries, although she said she would first have extensive consultation with U.S. allies. Aides said one difference is that Sen. Clinton will take her first year in office to repair ties with allies while studying the best ways to approach Damascus and Tehran.

“She won’t be filling up her address book during the first year to meet with Ahmadinejad,” said Lee Feinstein, a chief foreign-policy adviser to Sen. Clinton.

Middle East experts said Obama’s strategy holds potential pitfalls. In Iran, they said, Sen. Obama could strengthen Mr. Ahmadinejad if as U.S. president he moves too quickly to hold direct talks with Tehran’s leader. They note Mr. Ahmadinejad is facing presidential elections in 2009 and could use a summit with Sen. Obama as proof of his enhanced stature. They said Mr. Ahmadinejad also could seek to sell to his people that talks with Washington were a direct result of his hard-line stance.

“If Obama comes into office in January 2009, I wouldn’t advise him” to hold talks with Mr. Ahmadinejad quickly, said Karim Sadjadpour, an Iran specialist at Washington’s Carnegie Endowment for International Peace, who said he is generally supportive of Sen. Obama’s agenda. “Only two things can rehabilitate Ahmadinejad politically: bombing Iran or major efforts to engage” him ahead of the vote.

Sen. Obama has pledged his support for Lebanese sovereignty in the face of what is seen as extensive efforts by Syria to undermine Beirut’s political process. He also has pledged his commitment to United Nations resolutions calling for the disarming of the Lebanese militia Hezbollah, of which Syria is seen as the main arms supplier.

“Saying you’ll talk to Syria no matter what undercuts Washington’s position,” said Emile El-Hokayem, a Middle East expert at the Henry L. Stimson Center, a nonpartisan Washington think tank. “I don’t think it’s feasible to revolutionize how diplomacy is conducted” with Damascus.

Still, Mr. El-Hokayem said he believes Sen. Obama’s leadership could have a profound impact on the Middle East. “Switching from a hawkish leader to a charismatic one will have a huge impact” on Washington’s perception in the region, he said.

Sen. Obama’s aides disputed the charge that he would recklessly move into talks with Washington’s adversaries. Still, they said Washington doesn’t have the luxury to wait indefinitely to hold talks with the likes of Tehran or Damascus because of the depths of the instability in the Middle East.