Mitt Romney isn't making it easy to figure out his choice for running mate.

He's been very cagey and tight-lipped on the subject, while campaigning around the country with prominent Republicans believed to be in the running. Were the trips auditions?

The clock is ticking. The Republican National Convention in Tampa, Fla., is under three weeks away.

With Romney's wife, Ann, now back from London where she watched her horse Rafalca compete in the Olympics, the running-mate decision could come any day.

Romney on Saturday begins a four-state bus tour, with stops in Virginia, North Carolina, Florida, and Ohio — swing states where the race is hotly contested.

Three Republicans seen as vice presidential potentials will join him for various legs — Virginia Gov. Bob McDonnell and Sens. Marco Rubio of Florida and Rob Portman of Ohio.

Is this still another try-out or will a decision have been reached by then?

Others mentioned in speculation, including South Carolina Gov. Nikki Haley, New Mexico Gov. Susana Martinez, former Secretary of State Condoleezza Rice and former Florida Gov. Jeb Bush, already have been given convention speaking slots.

Romney campaigned Wednesday in Iowa, another key swing state, with a morning appearance at a Des Moines high school where he pressed an accusation that President Barack Obama had gutted a Clinton-era welfare law's requirement that recipients work — a claim both the White House and former President Bill Clinton rejected as false.

At issue is a recent Obama executive order giving states more flexibility in putting people back to work.

Obama, who'll spend three days next week touring Iowa by bus, was campaigning Wednesday and Thursday in Colorado — yet another battleground state — with his first stop in Denver.

After Iowa, Romney goes to New Jersey Wednesday to raise money.