Emotional pleas from former Rep. Gabrielle Giffords, D-Ariz., and her husband, Mark Kelly, on Tuesday tugged at senators both for and against a bipartisan proposal on background checks for gun purchases. It’s not clear whether they turned any votes, however, as negotiators were still scrambling to clear a 60-vote hurdle necessary for passage. Vice President Joseph R. Biden Jr. said Democrats were two votes short of a filibuster-proof majority on an amendment that would end the gun show exemption and expand background checks to online sales. Democratic aides tracking the bill said their magic number was a bit higher and...