A coalition opposed to overseas trade agreements is building grassroots support, gathering more than half a million signatures and making tens of thousands of calls to lawmakers to argue against trade legislation in Congress.

Unions, environmental groups, and political organizations—working under the umbrella site StopFastTrack.com—have gathered nearly 600,000 supporters through electronic petitions and similar efforts and made more than 40,000 phone calls to Congress. They are opposed to a trade measure, known as fast track or trade promotion authority, that aims to smooth congressional passage of trade deals currently being negotiated in Asia and Europe.