These days, political conventions are no longer where the two major parties select their presidential candidates. The selection process purports to be a democratic one, in which the grass roots have the opportunity to choose a candidate through the complicated series of state primaries and caucuses, although the reality is that the grass roots is usually choosing between one and three candidates preapproved by the party establishment.
While in 2008, Barack Obama did manage to upset Hillary Clinton, not even the complete lack of enthusiasm for Mitt Romney on the part of the Republican grass roots was enough to permit any of his nominal competitors for the nomination to effectively compete with him in 2012.