Barack Obama says he's chosen his running mate - but he's not telling who it is.

"I did say that I've made the selection and that's all you're going to get," Obama told reporters while campaigning in Virginia Thursday afternoon.

The presumptive Democratic presidential nominee earlier told USA Today that he has made up his mind on his No. 2, with the Democratic National Convention just days away. He would not say whether he's told that person.

"I won't comment on anything else until I introduce our running mate to the world," he said. "That's all you're going to get out of me."

Obama is expected to head to Springfield, Ill., Saturday for a joint appearance with his running mate. It's not clear whether he'll announce his selection before the weekend.

"We had some great choices," Obama told USA Today, adding he was looking for someone "prepared to be president."

Delaware Sen. Joe Biden, Indiana Sen. Evan Bayh, Virginia Gov. Tim Kaine and Kansas Gov. Kathleen Sebelius are seen as top prospects.

While Kaine was on the campaign trail with Obama on Thursday, Bayh and Biden have been on near-constant surveillance by reporters, who are tracking every move.

Biden was initially playful with reporters, feeding them coffee and bagels Wednesday while acting cagey about whether he's being considered by Obama.

But on Thursday he snapped at a reporter who was in a car apparently too close to his property.

"Get the car off my grass, pal," he said. "Now."

Biden brought no food to reporters.

Meanwhile, West Virginia Gov. Joe Manchin said Thursday that he thinks Kaine is on the short list. He spoke to Kaine about the vice presidential search Wednesday, and said Kaine told him although he hadn't heard anything from the Obama campaign on where he stands at the time, "he really thinks he has a chance at the short straw."

Obama and met with Kaine and his staff for 15 minutes Thursday morning at a Richmond hotel. Afterward, Kaine said he would let the Obama campaign speak about whether the candidate asked him to be his No. 2.

Obama is planning to announce the choice in a text message to supporters sometime before Saturday afternoon, when he's scheduled to appear with his pick in Illinois.

Asked when the text would be sent, Obama just grinned and said, "Wouldn't you like to know?"

Click here to read the article on Obama's interview in USA Today.

FOX News' Aaron Bruns and The Associated Press contributed to this report.