Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and of the new book "Governing America."

(CNN) -- Now the party is really starting. Democrats and Republicans are preparing to gather to hold their conventions, each using this precious time to tell the nation what its presidential candidate is all about.

Republicans are hoping that Gov. Chris Christie can tear down the Democrats, New Jersey style, in his keynote address, and that Condoleezza Rice can add some foreign policy heft to a ticket remarkably thin on international affairs. Democrats are depending on former President Bill Clinton to tap into the rhetoric he used against Republicans in the budget battles of 1995 to cut into Mitt Romney and Paul Ryan's vision for Medicare. They hope that San Antonio Mayor Julian Castro, the keynote speaker, can send a message to Latinos about which party is on their side.

Without any more deal-making in smoke-filled rooms, speeches are the highlight of the convention. Even when speeches are made at conventions whose candidate winds up losing, they can offer ideas and rhetoric that become integral to the party for decades to come. A look back at history reveals that there are different types of speeches that we might see in the coming weeks, each with very different purposes and effect....