Representatives of the of the AFL-CIO and United Steelworkers didn't return messages or declined to comment on their recent interest in patent reform. But the AFL-CIO letter alluded to recent efforts of the U.S. government and the U.S. tech industry to prod China to strengthen its intellectual property protections.

These two changes "may have a negative impact on innovation and research," the AFL-CIO letter said. "At a time when the Chinese government is constantly being challenged to live up to its intellectual property obligations, we do not want to take actions that may weaken ours."

The United Steelworkers letter said nearly 70 percent of patents filed in the U.S. come from manufacturing firms. "These bills would allow an endless loop of legal challenges after patents are awarded that will make it more difficult for U.S. patent holders to prevail against frivolous challenges," the letter said.

ITI's Ackil and representatives of two other tech groups said they doubted the unions' opposition would kill patent reform, although Ackil acknowledged that "anyone should be concerned" with opposition from large labor groups.

Ackil and a representative of another tech trade group, who spoke on background, questioned the depth of the opposition from the labor unions. "The AFL-CIO has been nowhere on this issue for three years," Ackil said.

Democrats knew of the labor concerns before the House Judiciary Committee approved the patent reform bill, added a spokesman for Representative Howard Berman, a California Democrat and primary sponsor of the House patent bill.

"We’re working now to try to understand and deal with any concerns labor may have," said spokesman Gene Smith. "We think that labor will not be opposing the bill when it comes to the floor soon after the House returns from recess."

But the labor opposition could lead to changes in the bills, said Bill Mashek, a spokesman for the Coalition for 21st Century Patent Reform, a group that has opposed parts of the current patent bills.

"Labor’s voice in the debate is significant and influential," Mashek said. "It shows, in our view, that the bill needs further work to ensure that U.S. manufacturing and innovators have the incentives to invest and develop new products. The concerns of ... organized labor resonate with members across the country and of all political stripes."