WASHINGTON—The Group of Seven (G-7) leaders rallied around a global tax deal last week in an effort to make big multinational companies pay their fair share of taxes. The deal met with swift opposition from Republicans, who called it “tax surrender” for the United States. Studies show that U.S. companies are likely to see the greatest impact on their taxes under the new proposal. Leaders of the world’s seven richest nations on June 13 endorsed a plan that would allow governments to tax large companies more broadly and implement a global minimum tax of 15 percent. The proposal, however, faces a roadblock in Congress. It may require changes to international tax treaties, which need bipartisan support. Several Republican lawmakers have already opposed the G-7 agreement. “If the policy has to be adopted through a new tax treaty, which I think is likely, then you will need a two-thirds majority in …