Nearly 10,000 of the biggest donors to Republican candidates and causes across the country will probably receive a foreboding “warning” letter in the mail next week.

The letter is an opening shot across the bow from an unusual new outside political group on the left that is poised to engage in hardball tactics to prevent similar groups on the right from getting off the ground this fall.

Led by Tom Matzzie, a liberal political operative who has been involved with some prominent left-wing efforts in recent years, the newly formed nonprofit group, Accountable America, is planning to confront donors to conservative groups, hoping to create a chilling effect that will dry up contributions.

“We want to stop the Swift Boating before it gets off the ground,” said Mr. Matzzie, who described his effort as “going for the jugular.”