Why is Alger Hiss' guilt such a threat to the liberal vision?

Mark Gauvreau points to this strange passage in last week's Washington Post:

The House Un-American Activities Committee investigated Hiss, launching the career of Richard M. Nixon, a committee member who pressed hard for Hiss's indictment, records would later show. The case was fodder for conservatives bent on portraying liberals as soft on Communism and therefore unpatriotic -- not an unfamiliar template for political combat in the modern era.

It also opened a deep fissure within American liberalism that reverberates to this day.

"Was it going to be the liberalism of the Franklin Roosevelt stripe, the New Deal vision of a communitarian society that takes care of its own and the poor, or was it going to be sort of a neo-liberalism that stood up to the Communists and turned its back on the New Deal vision?" says Kai Bird, who, with Martin J. Sherwin, authored the Pulitzer Prize-winning "American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer."

Why do they see that as their choice, asks Mr. Gavreau? Why cannot they be liberals who are also opposed to Stalinist totalitarianism? Why do they think that opposing Stalinism would betray the "New Deal vision"?

As the Washington Post unwittingly points out, these questions reverberate today: why do liberals think that opposing Baathism and Islamism would be a betrayal of their principles? The answer, I speculate, is that their vision requires them to imagine a unified world in which there is no enemy but Republicans. Thus, if Assad in Syria opposes Republicans, that, in Speaker Pelosi's eyes, is sufficient to make him one of the good guys who has a commitment to "peace." For the liberal "idealists," imagining a world with more than two factions, being liberal good guys and evil Republicans, would require a painful re-thinking.

Because the statute of limitations had run out, Alger Hiss was never tried for espionage. He was, however, convicted of perjury. His guilt was established beyond the slightest doubt by the exhaustively researched book, Perjury, by Allen Weinstein.