April 17, 2013 Article originally written at foxnews.com. The Senate has defeated a vital amendment seen as the linchpin to Democrats' gun control bill, dealing a major setback to President Obama as he campaigns to expand the federal background check system. The vote was 54-46, with supporters falling six votes short of the required 60-vote threshold. The Senate is proceeding to several other amendments, but the failure of the background check proposal authored by Sens. Joe M…