The presidential candidates traded barbs on Medicare and the economy in three battleground states Monday, as the campaign for the White House sped into a sprint.

The day mixed debate on the central fiscal issues facing Washington with traditional on-the-ground campaigning at venues including the Iowa State Fair in Des Moines, where Rep. Paul Ryan, in jeans and cowboy boots, made his first solo appearance since joining the GOP ticket. Across the state at a family farm in western Iowa, President Barack Obama, also in casual clothes, pledged aid to farmers suffering from a drought that has slashed crop yields.

Much of the discussion focused on the financially shaky Medicare health program for seniors, an issue of new prominence since Mitt Romney named Mr. Ryan as his running mate Saturday on the Republican ticket. The Wisconsin representative has been a favorite of fiscal conservatives from his role in Congress of promoting deficit-cutting plans that would overhaul entitlement programs. The campaign will now test how the broader electorate views those plans, including proposals to substantially change Medicare that align with Mr. Romney's platform.

Mr. Romney on Monday appeared in retiree-rich Florida and said he would strengthen Medicare. "We want to make sure that we preserve and protect Medicare," Mr. Romney said in St. Augustine.

Mr. Romney would offer retirees, starting in 2022, a choice between traditional Medicare or the option of buying private health insurance, with premiums supported by the U.S. government. He eventually would raise the Medicare eligibility age of 65 by one month each year.