All Odds Aside, G.O.P. Girding for Floor Fight By JEFF ZELENY and JIM RUTENBERG CHICAGO — For the first time in a generation, Republicans are preparing for the possibility that their presidential nomination could be decided at their national convention rather than on the campaign trail, a prospect that would upend one of the rituals of modern politics. The race remains Mitt Romney’s to lose, and if he continues to accumulate delegates at a steady clip starting with contests in Puerto Rico on Sunday and Illinois on Tuesday, he can still amass the 1,144 necessary to secure the nomination before...