WHEN THE FRINGE IS THE MAINSTREAM…. Dana Milbank had a very worthwhile column today, noting, among other things, the larger significance of a billboard in Mason City, Iowa.

You’re no doubt familiar with it by now. This is the billboard “depicting three leaders: Adolf Hitler (with swastika), Vladimir Lenin (with hammer and sickle) and Barack Obama (with 2008 campaign logo). Over Hitler were the words ‘National Socialism,’ over Lenin was ‘Marxist Socialism’ and over Obama was ‘Democrat Socialism.’”

The billboard was sponsored by local Tea Party activists, who eventually backpedaled, not because they realized their sign was insane, but because the media coverage made them look bad. The importance is the larger context: “The vile sign … was a logical expression of a message supported by conservative thought leaders and propagated by high-level Republican politicians.”

Quite right. Milbank notes, for example, a recent screed from Thomas Sowell, who equated the Obama presidency with the rise of Hitler. It was quickly touted by a right-wing member of Congress and a certain former half-term governor of Alaska.

These sentiments have long existed on the fringe and always will. The problem is that conservative leaders and Republican politicians, in their blind rage against Obama these last 18 months, invited the epithets of the fringe into the mainstream. […]

Consider these tallies from Glenn Beck’s show on Fox News since Obama’s inauguration: 202 mentions of Nazis or Nazism, according to transcripts, 147 mentions of Hitler, 193 mentions of fascism or fascist, and another 24 bonus mentions of Joseph Goebbels. Most of these were directed in some form at Obama — as were the majority of the 802 mentions of socialist or socialism on Beck’s nightly “report.” […]

Isn’t there a grown-up to rein in these backbenchers when they go over the top? Don’t ask House Minority Leader John Boehner, the man who would replace Nancy Pelosi as speaker. He accuses the Democrats of “snuffing out the America that I grew up in” and predicts a rebellion unlike anything “since 1776.” Boehner also said one Democratic lawmaker “may be a dead man” for his vote on health care and predicted that the bill would bring “Armageddon.”

The speed with which conservatives went from zero to hysterical in 2009 was impressive, but it’s the mainstreaming of sheer madness that doesn’t get the attention it deserves. As Milbank noted, “[A]ccusations that once were beyond the pale — not just talk of Nazis and Marxists but intimations of tyranny, revolution and bloodshed — are now routine.”

And they’re common, not just among fringe media personalities and activists, but with the Republican Party establishment.

It’s tempting to think there will be eventually be a Joseph Welch moment, but no one in the party seems willing to step up and acknowledge that Republicans shouldn’t follow the orders of unhinged zealots. On the contrary, they’re afraid to disappoint the radical base, which may in turn undermine the GOP’s “enthusiasm gap” edge.

And so the Republican Party shows no meaningful qualms about becoming the party of conspiracy theories (“Birthers,” Gulf oil spill was deliberate), wild-eyed accusations (ACORN, “re-education camps,” Gestapo-like security forces, New Black Panther Party), and radical policy positions (a five-year spending freeze to address a global economic crisis, the belief that tax cuts pay for themselves, a freeze on federal regulations, willful ignorance about energy, health, and education policy, the entire Sharron Angle/Rand Paul platform).

Rage and paranoia are not an attractive combination, but they’re driving the GOP talking points and the larger political discourse. So, when a member of the Republican leadership talked about the GOP emulating the Taliban, no one in the party deemed this controversial. When Republicans regularly compare U.S. leaders to Germany in the 1930s, the party mainstream barely bats an eye. When GOP policymakers openly discuss the prospect of state nullification of federal laws, no one in the Republican ranks steps up to say, “Good Lord, these people are mad.”

Best of all, Republican “leaders” are content to keep it this way. Indeed, it seems to be the centerpiece of the midterm election strategy.

The point isn’t that political radicalism is new; it’s clearly not. Rather, the key development over the last 18 months is the ways in which right-wing extremism has gone mainstream — with the consent of the Republican Party, which sees the electoral benefits of blind rage and fear.

That Mason City billboard warned the public that “radical leaders prey on the fearful & naive.” On this, the Tea Partiers were far more correct than they probably realize.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.