"O God, humiliate Bush and his party, O Lord of the Worlds, degrade and defy him," Abu Yahya al-Libi said at the end of sermon marking the Muslim feast of Eid al-Fitr, in a video posted on the Internet.
Libi, a top al-Qaeda commander believed to be living in Afghanistan or Pakistan, called for God's wrath to be brought against Bush equating him with past tyrants in history.
The remarks were the first from a leading al-Qaeda figure referring, albeit indirectly, to the US elections. Muslim clerics often end sermons by calling on God to guide and support Muslims and help defeat their enemies.
Terrorism monitor SITE Intelligence Group said in a report on Wednesday that militants on al-Qaeda-linked websites have for months been debating the significance of Democratic presidential candidate Barack Obama or Republican John McCain.
Some posters have also argued over the merits of trying to attack the United States before the election or waiting until later, the report said.
But SITE said it did not expect al-Qaeda leader Osama bin Laden or deputy Ayman al-Zawahri to openly favor a candidate.
"To support a particular candidate would debase al-Qaeda's long-standing argument that the United States government is a corrupt institution no matter who is at the helm," SITE director Rita Katz said in the group's November newsletter.
In 2004 bin Laden issued his first video in more than a year just days before the US elections. It derided Bush and warned of possible new September 11-style attacks.
Bin Laden made little mention of Bush's Democratic challenger, John Kerry, telling Americans: "Your security is not in the hands of Kerry or Bush or al-Qaeda. Your security is in your own hands and each state which does not harm our security will remain safe."
Kerry has attributed his loss in part to the video's high-profile reminder of the terrorism issue.
In 2006, after Democrats captured Congress, Zawahri issued an audio message saying all Americans remained al-Qaeda's enemies regardless of party, SITE said.
SITE said militant postings on al-Qaeda-linked websites typically discuss Obama in terms of his race, or his religion and foreign policy. Some forecast a racial crisis dividing the United States if he wins. Others say his planned phased withdrawal from Iraq would be a boon to al-Qaeda's affiliate and give it a base for Middle East expansion.