The group, Change to Win, says Mark Penn should be removed from Clinton's campaign team for meeting with the Colombian ambassador about a trade agreement that is opposed by many Senate Democrats, The Hill reported.
The newspaper said Penn met with the ambassador in his capacity as head of the lobbying firm Burson-Marsteller Worldwide.
"It's time for Senator Hillary Clinton to send her vaunted 'chief strategist' Mark Penn packing - back to his job consulting for union-busting corporations and anti-labor governments for good," Greg Tarpinian, executive director of Change to Win, said in a statement.
Penn has said he regrets the meeting.
"The meeting was an error in judgment that will not be repeated and I am sorry for it," Penn said. "The senator's well known opposition to this trade deal is clear and was not discussed."
The flap comes a week after a strategist for Republican presidential candidate John McCain resigned from his lobbying firm to avoid the appearance of impropriety.
"I always thought it was easy for me to treat a presidential candidate as a separate client," strategist Charlie Black told The Politico. "But I do understand that some people consider it a conflict or potential conflict of interest."