MARTINSVILLE, Va. -- Barack Obama is sounding tough populist themes on the campaign trail, pledging to create union jobs to build energy-efficient cars and to end tax breaks for corporations that ship jobs overseas.

Obama is giving no hint of when to expect the announcement of his vice presidential choice.

The presumptive Democratic nominee is on a bus tour in Virginia, a state that has favored Republicans in recent presidential elections but is viewed as a battleground this year.

In Martinsville, on Wednesday, Obama renewed his warning that Republicans would try to scare voters about his background.

Obama is traveling with former Virginia Gov. Mark Warner, who will serve as the keynote speaker at the party's convention in Denver next week.