WASHINGTON, April 5 (UPI) -- A coalition of unions backing Democratic U.S. presidential hopeful Barack Obama is calling on rival Hillary Clinton to fire one of her advisers, the group said.

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."