THE DENVER THREE….The Bush White House’s “Bubble Boy” policies — which, for years, have excluded anyone from presidential events that isn’t a pre-screened sycophant — have become the stuff of legend. But there’s always been something a little different about the Denver Three.

In March, Denver residents Alex Young, Karen Bauer, and Leslie Weise obtained tickets from their Republican congressman to a public town hall meeting on the president’s Social Security plan. Someone working at the event noticed an anti-war bumper sticker (“No Blood For Oil”) on their car, which prompted staffers to forcibly remove the three from the presidential event, despite the fact that they hadn’t done anything wrong.

Even for a White House known for shielding the president from potential critics, this was bizarre. There are plenty of examples of people being excluded from presidential events for being Democrats. Others, because their shirts or lapel stickers were deemed ideologically unacceptable. But this was an example of American citizens getting escorted out of a public event, dealing with a public policy issue, on public property, featuring public officials, because someone didn’t like their bumper sticker.

Young, Bauer, and Weise, who quickly became known as the “Denver Three,” left the event quietly, but have been anything but silent since. The Secret Service launched an investigation of the incident, exploring whether someone impersonated an agent, but concluded the probe in July without explanation. The Denver Three have filed FOIA requests with the White House, but they were ignored.

Today, two of the Denver Three, with assistance from the ACLU, are taking the next logical step. They’re taking the matter to court.

White House event staffers unlawfully removed two Denver residents from a town hall discussion with President Bush because of an anti-war bumper sticker on their car, charged the American Civil Liberties Union in a federal lawsuit filed today.

“The government should not be in the business of silencing Americans who are perceived to be critical of certain policy decisions,” said ACLU Senior Staff Attorney Chris Hansen, who is the lead counsel in this case. “The president should be willing to be in the same room with people who might disagree with him, especially at a public, taxpayer-funded town hall.”

It’s a lawsuit that could raise a variety of interesting, and potentially damaging, questions for the Bush White House.

For example, does the White House have a formal policy for evicting law-abiding ticket-holders from public events? Who gives directions to event staffers about their responsibilities? How are people working at these events recruited and trained? Are they specifically told to engage in viewpoint discrimination? Does the White House encourage this approach?

In June, Sen. Wayne Allard’s (R-Colo.) chief staff said, the Denver Three are “entitled to some answers.” If this lawsuit is successful, they’ll get them.

Post Script: It’s probably worth taking a moment to debunk the White House’s likely response. Asked about the controversy a few months ago, Scott McClellan said the Denver Three were ejected out of concern “that these three individuals were coming to the event solely for the purpose of disrupting it.” The three admit that they had considered creating an incident during Bush’s speech, but decided against it.

Regardless, whatever plans the Denver Three may or may not have made beforehand, the argument about concern for disruption is silly. These three were given free tickets to see the president. There was nothing wrong with their attire, they hadn’t said a discouraging word to anyone, and there was no disturbance. In this case, the Bush White House lowered the bar so far that someone didn’t even need to disrupt the event to get thrown out; staffers merely had to believe someone might cause trouble based on a bumper sticker. In a nation that takes free expression seriously, this is absurd.

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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.