LIMA, Ohio — Mitt Romney's running mate Paul Ryan rolled out Monday on a three-day bus tour across Ohio, a battleground state seen as critically important for the Republican ticket's White House hopes.

Trailing President Barack Obama in national polls as well as in most swing states after a difficult week, Romney is seeking a campaign reset that puts him back on the stump for the final six-week push to the November 6 election.

Ohio and Florida are seen as the two monumental battlegrounds of 2012, and after a handful of events in the Sunshine State late last week, Romney will be joining Ryan on Tuesday and complete the final two days of the tour through the Midwestern state of Ohio.

"This is ground zero," Republican National Committee chairman Reince Priebus, acknowledging the importance of the Buckeye state, told a crowd of about 1,200 people in Lima as they awaited Ryan.

Ohio is pretty much a must-win for Romney; no Republican has ever been elected president without winning the state.

Romney has been criticized in recent days for spending more time hosting fundraisers than doing public campaigning, but in an interview with ABC News on Monday he said he and Ryan were "very anxious to get out and speak with as many people as we can."

"Both Paul and I, we hit the road pretty darn hard," Romney said in Colorado after a campaign appearance there.

Romney and Ryan will both campaign in Ohio on Tuesday, the first time the pair have appeared together at an event since September 1.

In Lima, Ryan bounded out on stage and quickly lashed Obama for four years of economic failure.

The US economy is "barely limping along, and the president has no idea how to grow it," the 42-year-old congressman from Wisconsin said.

"The contrast could not be clearer in this election," Ryan said, as he laid out the case for Romney, a multimillionaire investor who he said has the business know-how and experience to turn around the flagging economy.