Labor groups argue that allowing AT&T to buy T-Mobile will lead to a larger unionized workforce because AT&T employees are unionized, while T-Mobile's employees are not.
But Franken told the labor groups he believed the merger would destroy jobs and lead to higher phone bills for consumers.
"We know from their documents that some of their [AT&T's] claims on jobs may not be accurate," said Franken, who sits on the Judiciary Committee's Antitrust subpanel.
The merger is one of the few issues where Franken has found himself on the opposite side of organized labor.
"I do understand their position. If I were them, I would have the same opinion," Franken said. But, he added, "I represent all people in Minnesota. I believe in the labor movement but that doesn't mean that I don't consider everyone in Minnesota and this is such an important part of our economy."
The Federal Communications Commission and the Justice Department are currently reviewing the merger. On Friday, the FCC restarted the countdown to its self-imposed deadline to reach a decision on the deal.