White House spokesman Jay Carney responds to a question about the Boston Marathon bombing at the White House in WashingtonBy using John Whitesides and David Lawder WASHINGTON (Reuters) – The centerpiece of President Barack Obama's campaign to curb gun violence was once on shaky ground on Wednesday as the Senate headed toward votes that might make or smash probably the most vital push for gun restrictions in the United States in 20 years. Despite emotional pleas from families of victims from the Newtown bloodbath and huge public toughen nationwide, a bipartisan deal to expand heritage checks for gun buyers gave the look to be in need of the 60 Senate votes wanted to clear procedural hurdles put in position via Republicans. …