American political conventions are now principally talent shows. In distributing the coveted primetime speaking slots, the parties showcase their future stars and give a chosen few their first taste of national celebrity.

A boffo performance can make a career, as Barack Obama’s stunning 2004 keynote did. A lousy one can do just the opposite, as New Jersey governor Chris Christie’s hot-headed and self-regarding speech seems to have done last week in Tampa.

This year, both parties sent a strong message with their selection of up-and-comers: we’ve got the bright, young Latinos. On the final night of the Republican convention, Florida senator Marco Rubio had the honour of introducing Mitt Romney. On the first night for the Democrats, the even younger and more charismatic San Antonio mayor Julian Castro opened the final hour. The Rubio-Castro matchup frames a crucial competition: the battle for an electoral segment that could determine the election and will only become more important thereafter.