Republican presidential contender Mitt Romney will kick off a four-day bus tour of swing states Saturday in Virginia, perhaps the most important battleground this fall.

Joining Romney on the trail will be Virginia Gov. Bob McDonnell, long speculated to be a top contender for Romney's vice president.

Unless Romney announces his vice presidential pick soon - and there is speculation that he will - McDonnell's presence on Saturday is likely to further fuel speculation that he's in the mix for the job.

Romney's stops in Virginia will include Manassas. After Virginia, he'll head for several other battleground states, including North Carolina, Florida and Ohio. Sen. Marco Rubio, R-Fla., and Sen. Rob Portman, R-Ohio, are also considered possible vice presidential nominees.

President Obama has visited Virginia at least twice over the last month.