Sen. John McCain (R-Ariz.) has emerged as a key player if Senate Democrats are to have any chance of passing legislation to expand background checks for private sales of firearms.McCain and Sens. Susan Collins (R-Maine) and Dean Heller (R-Nev.) are at the top of a list of Republicans considered most likely to sign on to legislation expanding background checks after talks with Sen. Tom Coburn (R-Okla.) stalled earlier this month.