This hits close to home for me and for many politically involved Democrats in the state as I frequently attend meetings there and used to live about three blocks away.
Remarkably, "Police said the person responsible is in custody." This is unusual because vandalism cases frequently go unsolved. My guess is that someone was either caught in the act, or was caught on videotape (perhaps a traffic camera or video from a nearby convenience store).
According to the Denver Post:
The Denver Police Department has a man in custody, though they are not yet releasing the suspect's name, said Det. Vicki Ferrari.
An officer on patrol spotted the vandal in the act around 2:20 a.m. and took him into custody after a short foot pursuit, she said.
Redstateblues, a sometimes reader, has posted a photo of an anti-health care flier ("National Socialist Healthcare" reads the headline) from the scene at Colorado Pols.
So, we may yet get a definite answer regarding the motive for the crime, which Colorado Democratic Party chair Pat Waak suggests was related to opposition to the President's health care plan.
I'll also be a first to say that this matter should be handled as criminal vandalism and not as some sort of political offense (e.g. terrorism). Dignifying the cause of someone who engaged in property destruction with a purpose doesn't help.
UPDATE #2 (first update reflected in original text): The perpetrator who was caught appears to be Maurice Schwenkler, age 24, who isn't registered to vote, who did some election work for a 527 group that favored Democrats, shortly before the election in 2008, "signed an online 2005 petition to free anti-war Christian protestors who were captured in Iraq" and is active in the Derailer Bicycle Collective (the collective's address is what he cites as his address in the contribution report; it is in the same neighborhood as the Colorado State Democratic Party HQ). The Collective is a bit of an anarchist leaning random acts of kindness liberal group. In other words, he's a usually harmless hippie.
Another suspect escaped the scene on a bicycle.
Clearly, this particular incident was not a case of right wing political violence. It may actually have been an attack calculated to engender sympathy for the Democrats, or it could have been some sort of more personal grievance.