TO CONSPIRACY theorists like Rep. ­Michele Bachmann (R-Minn.), the Obama administration’s approach to the Arab world is the product not of considered diplomacy but of wicked “influence operations,” traceable to the Muslim Brotherhood and its agents. Exhibit A among those agents with murky “ties” to the Muslim Brotherhood, Ms. Bachmann warns darkly, is Huma Abedin, a longtime aide to Secretary of State Hillary Rodham Clinton.

Ms. Bachmann’s smear of Ms. Abedin, a 37-year-old Muslim American born and educated in this country, was contained in a letter last month to the State Department’s inspector general’s office. It would be simple to ignore the baseless and paranoid assertions of Ms. Bachmann were she not a member of Congress and an also-ran in the recent race for the Republican presidential nomination. Her status doesn’t confer respectability on her views — Americans are inured to all manner of nonsense from Congress — but it does call for a response, if only to restore a dose of rationality to the public discourse.

That response was delivered effectively Wednesday on the floor of the Senate by John McCain, an Arizona Republican who knows Ms. Abedin, as well as what it means to be slimed in public life. Mr. McCain, who as a candidate for the GOP presidential nomination in 2000 was subjected to malicious and false rumors that he had fathered a child out of wedlock, spoke with feeling about Ms. Abedin.

“Huma represents what is best about America,” the senator said. She is “the daughter of immigrants, who has risen to the highest levels of our government on the basis of her substantial personal merit and her abiding commitment to the American ideals that she embodies so fully.”

Ms. Bachmann’s letter, signed by four other Republican congressmen, asserts that Ms. Abedin’s father (who died when she was a teenager), mother and brother are “connected” to the Muslim Brotherhood. It calls on the State Department’s inspector general to investigate the organization’s supposed influence in the U.S. government. In a separate letter, Ms. Bachmann asks how Ms. Abedin, who is Ms. Clinton’s deputy chief of staff, received her security clearance.

Ms. Bachmann’s accusations are tissue-thin garbage of the someone-said-something variety — or, as Mr. McCain put it, “nothing less than an unwarranted and unfounded attack on an honorable woman, a dedicated American and a loyal public servant.”

Among the co-signers of Ms. Bachmann’s letter are Rep. Trent Franks of Arizona, notorious for describing abortion as having done more harm to blacks than slavery; Rep. Lynn Westmoreland of Georgia, who described then-presidential candidate Barack Obama and his wife, Michelle, as “uppity”; and Rep. Louie Gohmert of Texas, known for evidence-free rants about plots involving U.S. passport-bearing “terror babies,” born here and trained overseas to carry out attacks on America.

Spurred or shamed by Mr. McCain’s courageous remarks, some Republican leaders, but not all, have gathered themselves to repudiate Ms. Bachmann and her co-signers. On Thursday, House Speaker John Boehner (R-Ohio) praised Ms. Abedin’s “sterling character,” noting that the accusations against her are “dangerous.”

Ms. Bachmann’s accusations are in the squalid tradition of McCarthyism and other shameful chapters of our history in which politicians have slandered their rivals as un-American, allied with enemy forces or guilty by association with shadowy forces. Her tactics are no less ugly than those of her predecessors. Thanks to Mr. McCain, they may be less effective.