By Doina Chiacu WASHINGTON (Reuters) -Republican Senate negotiators on an infrastructure deal were optimistic about a $1.2 trillion bipartisan bill on Sunday after President Joe Biden withdrew his threat to veto the measure unless a separate Democratic spending plan also passes Congress. U.S. Senator Rob Portman said he and his fellow negotiators were “blindsided” by Biden’s comments on Thursday after a rare bipartisan compromise to fix the nation’s roads, bridges and ports. “I was very glad to see the president clarify his remarks because it was inconsistent with everything that we had been t…