U.S. Senator Carl Levin

U.S. Senator Carl Levin

Andrew Harrer/Bloomberg

Under the plan by U.S. Senator Carl Levin, a Democrat from Michigan, U.S. companies trying to buy a foreign company and relocate their headquarters to a lower-tax country would have to ensure that shareholders of the non-U.S. company own at least 50 percent of the combined company, up from 20 percent now.