Despite the to incumbent hardline President Mahmoud Ahmadinejad, many officials and experts believe a Mousavi victory will result in only incremental shifts toward the U.S.
Because real power in Tehran is still wielded by religious leader Ayatollah Ali Khamenei, some say an Ahmadinejad re-election may, in fact, make it easier to build an international consensus against Iran.
"The argument is that if a more moderate president is elected, it may give the U.S. undue confidence about the character of the Iranian regime," said Karim Sadjadpour, an Iran analyst at the Carnegie Endowment for International Peace.
Ali Nader, an Iran expert at the RAND Corporation, said there are clear differences between Ahmadinejad and Mousavi but they may mean little in the Iranian government's behavior.
"The two candidates are different in terms of policy, but the supreme leader makes the ultimate decisions," he said. "If Mousavi wins, he won't necessarily be able to change foreign policy but he may be able to improve Iran's image internationally. He could realign policy to a limited extent but I wouldn't expect a great change."
Administration officials have remained silent out of concern that any comments might influence the results. But they are privately hoping for result similar to Lebanon's recent election, won by a western-backed moderate coalition.
Underlying the U.S. reluctance to speak out are Iran's as-yet undelivered responses to President Obama's overtures for engagement, which included his recent address in Cairo to the Muslim world as well as, earlier, a televised New Year's address to the Iranian people and a series of diplomatic contacts.
Officials say Mr. Obama's attempts to reach out have gone largely unanswered - not unexpected given the political uncertainty in Iran ahead of the election.
But that has left officials in a quandary over how to proceed.
"We're following the very lively debate that we're seeing going on inside Iran and we wish them success in their elections," State Department spokesman Ian Kelly said Wednesday, tiptoeing around questions about the vote's possible impact.
Once results are in, officials say, the administration plans to intensify its overtures as well as diplomatic discussions. Those efforts are aimed at persuading Iran to stop uranium enrichment activities that could produce the ingredients for a nuclear bomb.
Mr. Obama himself has set an end-of-year deadline for Iran to respond positively to those efforts or face new and stronger sanctions. Other officials have suggested that the timeframe could be earlier, pegging progress to the upcoming United Nations General Assembly session at the end of September.
However, some experts say an Ahmadinejad loss may buy Iran more time from the United States.
"If Ahmadinejad wins, there will be no transition and you will see the administration not wanting to waste more time on the negotiations and sanctions," said Carnegie's Sadjadpour, adding that Mr. Obama will also be looking for quick responses on offers to engage on Afghanistan, Iraq and other matters.
"If he loses, there will be a delay in discussing at least the nuclear issue as the new team comes on board."