WASHINGTON – Rushing toward their party conventions, the rival presidential campaigns are trying to invigorate core supporters while reaching out to a sliver of undecided voters who harbor doubts about President Barack Obama yet aren't sold on Republican Mitt Romney.

In the past week the campaigns have engaged in a vigorous debate over Medicare, pushing aside the economy and jobs, for the moment. Romney has charged Obama with running a campaign based on hatred, Obama has renewed a fight over Romney's tax returns, and the issue of government spending has blossomed again.

These are the August seeds that candidates are planting with fence-sitting voters even as the campaigns try to get backers excited in time for the conventions. Republicans will gather in Tampa, Fla., in a week, and Democrats will be in Charlotte, N.C., in the first days of September.