WASHINGTON (AP) — Vermont Sen. Bernie Sanders said Tuesday he has "zero doubt" that rank-and-file union members will support his Democratic presidential campaign even though Hillary Rodham Clinton has locked up many endorsements.

Sanders joined labor activists on Capitol Hill to discuss new legislation to allow workers to join unions. He told reporters after the event that process for picking up labor endorsements isn't over "by any means" and he had the support of many workers in organized labor.

"Secretary Clinton has a lot of contacts with many of the union leaders over the years. But I have zero doubt that we have massive rank-and-file support among trade unionists all over this country," Sanders said.

Clinton won the support of the National Education Association on Saturday and has notched eight labor endorsements representing about 7 million workers. Sanders was endorsed by the National Nurses United, which represents about 185,000 workers but many large unions have yet to signal their support, including the Service Employees International Union and the American Federation of State, County and Municipal Employees.

His legislation would allow a union to be certified by the National Labor Relations Board if a majority of eligible workers sign valid authorization card. Similar legislation has been blocked by majority Republicans in Congress.