Republican Senator John McCain leapt across the political aisle Wednesday to defend a top aide to Democrat Hillary Clinton who has been accused of conspiring with Egypt's Muslim Brotherhood.

McCain railed against accusations from five lawmakers in his own party, including former presidential candidate Michele Bachmann, that Huma Abedin is somehow part of a Brotherhood conspiracy to influence US foreign policy.

Abedin, who is in her mid-30s, is a long-time aide of Secretary of State Clinton who has traveled around the world with her and acted as her chief of staff during her losing 2008 run for the White House.

McCain's intervention, delivered on the floor of the Senate, was startling as it came against the backdrop of stark polarization in America's bitter politics, with the rival camps in Washington gearing up for November elections.

"Rarely do I come to the floor of this institution to discuss particular individuals," McCain said to open his remarks.

"But I understand how painful and injurious it is when a person's character, reputation, and patriotism are attacked without concern for fact or fairness. It is for that reason that I rise today to speak in defense of Huma Abedin."

McCain said he had come to know Abedin over many years as a devoted public servant whose "decency, warmth, and good humor" shone through as she "devoted countless days of her life to advancing the ideals of the nation she loves."

Abedin, a Muslim, was born in Kalamazoo, Michigan in 1976 but moved with her family to Saudi Arabia when she was two. Her father was born in India under British rule and her mother is Pakistani.

"Put simply, Huma represents what is best about America: the daughter of immigrants, who has risen to the highest levels of our government on the basis of her substantial personal merit," McCain said.

"Recently, it has been alleged that Huma, a Muslim American, is part of a nefarious conspiracy to harm the United States by unduly influencing US foreign policy at the Department of State in favor of the Muslim Brotherhood and other Islamist causes," he said.

Five members of Congress, including Minnesota congresswoman Bachmann, wrote a letter to the deputy inspector general of the State Department alleging the conspiracy and demanding a probe.

"These allegations about Huma, and the report from which they are drawn, are nothing less than an unwarranted and unfounded attack on an honorable woman, a dedicated American, and a loyal public servant," McCain said.

He went on to shoot down the accusations, explaining that one member of Abedin's family alleged to be part the conspiracy, her father, had actually passed away two decades ago.

"These sinister accusations rest solely on a few unspecified and unsubstantiated associations of members of Huma's family, none of which have been shown to harm or threaten the United States in any way," McCain said.

"These attacks on Huma have no logic, no basis, and no merit. And they need to stop now."

Abedin came under the spotlight last year when her husband, New York congressman Anthony Weiner, resigned after sending lewd online messages and photographs on his cell phone and then lying about it.

"The Secretary (Clinton) very much values her wise counsel and support, and we think that these allegations are preposterous," said a statement from the State Department following McCain's speech.

The United States voiced caution last month after the Brotherhood's Mohamed Morsi was crowned Egypt's new president, urging the Islamist leader to respect the rights of women and keep the peace with Israel.

The Muslim Brotherhood is an Islamist political movement founded in Egypt in 1928 and a staunch opponent of several Middle East governments that enjoyed US backing, notably Israel and the formerly secular regimes in Egypt and Tunisia.