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) -- The stakes are high for Republicans at this week's convention. As the delegates gather in Tampa,the GOP needs to be ready to lay out a set of compelling arguments to frame the final months of the campaign.

More Americans will be paying attention to the campaign with the start of the conventions. With a new CNN poll showing that the Republican Party's favorability rating is down to 43 percent, the convention needs to strengthen the party's brand name. And since the convention will be a day shorter because of Tropical Storm Isaac, the GOP needs to make sure that it uses its time well.

If Republicans don't get the arguments right, they could easily miss the post-convention bounce they are counting on to give Mitt Romney some momentum after a rough summer.

Republicans need to make a compelling case that President Barack Obama's policies have failed to revitalize the economy....