WASHINGTON — A Falmouth childcare provider upset with the U.S. Chamber of Commerce’s political activity in Maine and nationwide spoke at a Friday rally in Washington intended to pressure the powerful business group to disclose its donors.

“My message is simple: the U.S. Chamber of Commerce doesn’t speak for small business and it doesn’t speak for me,” Melanie Collins, owner of Melanie’s Home Childcare and a member of the organization Main Street Alliance, told a crowd of several dozen activists near the Chamber’s headquarters.

The U.S. Chamber has spent more than $1.3 million so far on political advertising in Maine’s U.S. Senate race – all of it opposing independent Angus King or supporting Republican Charlie Summers – and has pledged to spend as much as $100 million nationwide on the 2012 elections.

The U.S. Supreme Court’s Citizens United ruling opened the door for corporations and labor unions to spend unlimited amounts of money on elections. As a nonprofit, the Chamber is not required to disclose the donors behind its political activities. That has led to allegations that corporations are funneling huge sums into the election through the Chamber and other organizations, all the while leaving the public in the dark about who is behind the attack ads and other messages.

On Friday, representatives of several progressive organizations — the U.S. Public Interest Research Group, Public Citizen, the Business Ethics Network, Main Street Alliance and Citizens for Responsibility and Ethics in Washington – held a rally to call for stronger disclosure laws.