More than 11,000 donors to liberal causes recently opened their mailboxes to a rude surprise.

Inside was a letter warning these donors that their political involvement would be publicized "in your local community."

The letter, circulating since September, was written by Americans for Limited Government Foundation, a libertarianadvocacy group based in Fairfax, Va. It was sent to people who have donated to Barack Obama’s campaign and advocacy groups such as MoveOn.org and Planned Parenthood. (Donor information is available for public review at www.fec.gov).

"Your name has been put in our database," the letter (PDF) said. "You have been able to engage in these activities without notice, operating in relative obscurity. I am writing to inform you that this will no longer be the case."

The letter went on to vaguely threaten its recipients, putting them on notice that "instances of coordinated voter fraud are surfacing" and "investigations into possible criminal coordination are underway." Attached was a memo (PDF)--written by Washington D.C. law firm Webster, Chamberlain and Bean—that details penalties for election law violations.

The letter was signed by the chairman of Americans for Limited Government, Howard Rich. A New York real estate investor, Rich has been part of ballot initiative campaigns on issues such as property rights and political term limits. News organizations have scrutinized Rich’s political activity in several states, including South Carolina and Oregon.

The PBS program, NOW, explained:

Organizations associated with Rich have funneled nearly $7 million into 2006 state initiatives aiming to limit government in 12 states, according to an investigation by The Oregonian published last month. Rich has generally declined to reveal how much of the money comes from his personal wealth, and is not required by campaign finance laws to report how much he privately funds his various groups.

We called the Americans for Limited Government for comment but have yet to hear back.

Rich wrote that he was sending his letter in response to "attacks, slurs and threats" he has received for supporting groups "that I believe will push back against the radical agenda of the left."

Specifically, Rich referenced a letter sent to Republican donors in August by the left-wing group, Accountable America. This letter, sent to 10,000 of the biggest donors to Republican candidates and causes, warned that many supporters of advocacy groups have had their "business and personal affairs closely scrutinized by the media and well-funded watchdog groups."

Accountable America also is offering a $100,000 reward to encourage the reporting of information to authorities about conservative advocacy groups violating federal campaign finance laws.