As the Democratic primaries near their end, supporters of both Hillary Clinton and Barack Obama have used a time-honored yet unexpected device to attack each other: old-fashioned redbaiting.

At the Philadelphia presidential debate in April, George Stephanopoulos asked Obama about his relationship with the Weather Underground terrorist Bill Ayers, who with his comrades bombed several government buildings in the 1970s. Obama protested that he knew Ayers as a neighbor and professor of English (actually, he teaches education) whose "detestable acts" when Obama was eight were no reflection on "me and my values."

But as soon as it was her turn to speak, Obama's opponent piled on. Ayers and Obama had served together on the board of the Wood Foundation in "a paid directorship position," noted Clinton. It was legitimate to raise questions about their relationship, she insisted, since Ayers's bombings had resulted in people's deaths. This line of attack may have been shortsighted on Clinton's part, considering that her husband pardoned two imprisoned members of the Weather Underground before leaving office, but the Clinton campaign didn't back off.

Before you could say Comrade, Clinton's close adviser Sidney Blumenthal was emailing out blog posts, articles, and reports from a wide array of conservative sources. Blumenthal's missives went to "an influential list of opinion shapers--including journalists, former Clinton administration officials, academics, policy entrepreneurs, and think tankers," as the left-wing activist and professor Peter Dreier reported on the Huffington Post (May 1).

This was shocking in its own way. Blumenthal, the very man who coined the term "vast right-wing conspiracy," Dreier noted, by circulating articles from the conservative media, was attempting to exploit "that same right-wing network to attack and discredit Barack Obama."

Blumenthal sent out pieces from the ultra-conservative Accuracy in Media (AIM)--"With Obama, It's the Communism, Stupid," "Obama and the Fifth Column," "Is Barack Obama a Marxist Mole?"--as well as items from more mainstream conservative publications, such as a Fred Siegel cover story from National Review, Fred Barnes's "Republicans Root for Obama" from THE WEEKLY STANDARD, and an older City Journal article by Sol Stern reporting Bill Ayers's current role in developing a radical curriculum for K-12 teachers ("Ayers's texts on the imperative of social-justice teaching are among the most popular works in the syllabi of the nation's ed schools and teacher-training institutes").

Particularly grating to Obama supporters was Blumenthal's airing of AIM's allegation that Obama had sought to hide the influence a Communist mentor had on him as a young man. In his memoir, Dreams From My Father, Obama mentions a certain "Frank," a black poet friend of his white grandfather's who was a "contemporary of Richard Wright and Langston Hughes" and had once had "some notoriety." Frank gave the young Barack some "hard-earned knowledge" (such as that "black people have a reason to hate. That's just how it is"). As Obama set off for college, Frank told him that college was "an advanced degree in compromise" and that he should not "start believing what they tell you about equal opportunity and the American way."

It was easy for students of American communism to figure out that this was Frank Marshall Davis, a Chicago writer and Communist activist who moved to Hawaii in the late 1940s. That Davis sought to advise the young Obama as he prepared to leave home hardly proves that Davis was a major influence on Obama or that the young man accepted his Communist views. Obama's withholding of Davis's full name, however, does suggest that he worried it might cause him problems in his political career--as if Davis were another difficult uncle like Jeremiah Wright.

At one time, left/liberal people would have vigorously objected to all this redbaiting. But Obama's supporters responded in kind. Hadn't Clinton opened the door, as Bill Ayers's brother argued on the Huffington Post (April 17), by engaging in "the most base version of McCarthyism"? If Obama had left-wing connections in his youth, why not bring forward Clinton's own hidden past? Let's see who the real leftist is!

First to attack was New Left elder statesman Tom Hayden, who told readers of the Nation magazine's website (April 22) that Clinton herself had been as far left as one could get. And unlike Obama, she did not have the excuse of being eight years old when the New Left radicals were in their prime. Hayden revealed that Hillary "was in Chicago for three nights during the 1968 street confrontations" and that at Yale Law School in 1970 she chaired a meeting where students voted to join a national strike against the Vietnam war. The same year, during the trial of Black Panther leader Bobby Seale for murder, Clinton oversaw Yale law students who were following the proceedings and looking for signs of government misconduct. Most significantly, Hayden writes, Clinton went to work after law school for the San Francisco law firm that defended the Panthers, led by Robert Treuhaft, a former member of the Communist party.

Hayden, of course, sees these activities as "honorable" and asks a simple question: "Doesn't the Rev. Jeremiah Wright, whom Hillary attacks today, represent the very essence of the black radicals Hillary was associating with in those days?" Now she has become a "guilt-by-association insinuator," who is "engaged in a toxic transmission onto Barack Obama of every outrageous insult and accusation ever inflicted on her by the American right." Furious at this betrayal, Hayden calls her "Lady Macbeth."

Hayden's sally was followed by one from Clinton's biographer Carl Bernstein on the Huffington Post (May 2). What upset Bernstein was that Clinton was evading the truth about her own past radical activities and associations.

These began at Wellesley, Bernstein wrote, when "she exhibited an academic fascination with the Left and radicalism." Later at Yale she was associate editor of an alternative law review that depicted "policemen as pigs and murderers." Yet, notes Bernstein, in her 2003 memoir, Clinton breathed not a word of her activity on behalf of the Black Panthers, nor was she honest about why she went to work for the Robert Treuhaft law firm. Treuhaft told Bernstein that Clinton came to the firm because it was a "Movement law firm" and she was "in sympathy with all the Left causes." Treuhaft commented that back then, "we still weren't very far out of the McCarthy era." Bernstein adds, "And might still not be, to judge from the 2008 presidential campaign."

It is just as silly, Bernstein concludes, to tie Obama to the Weather Underground as it is to call Clinton a Stalinist. Yet Bernstein and the others have inadvertently opened up two legitimate lines of inquiry: What remains of their old radical ideals in both candidates' present thinking, and how far is each willing to go in exploiting the other's past? If scrutiny of these matters is fair game for them, it can hardly be off limits for the press and the voting public.