The top U.S. presidential candidates focused on policy Wednesday, as speculation continues over whom Barack Obama and John McCain will choose as running mates.

Democratic presidential candidate Sen. Barack Obama, D-Ill. speaks at a town hall meeting at Patrick Henry Community College in Martinsville, Va., Wednesday, 20 Aug. 2008

During a speech on the economy in the eastern state of Virginia, Democrat Barack Obama told voters his Republican rival's economic policies would simply continue what he described as the failed policies of President George Bush.

During a town hall meeting in the southwestern state of New Mexico, Senator McCain again criticized Obama for opposing expanded offshore oil drilling.

Senator Obama is expected to appear with his pick for vice president Saturday in the Illinois capital of Springfield, where he kicked off his White House bid in February 2007. The party's nominating convention begins Monday.

McCain may announce his running mate on his 72 birthday during a rally on August 29, three days before the start of the Republican National convention.

Obama's top contenders are said to be Delaware Senator Joe Biden, Indiana Senator Evan Bayh and Virginia Governor Tim Kaine.

McCain is said to be considering former Massachusetts Governor Mitt Romney, Minnesota Governor Tim Pawlenty, and Tom Ridge, a former Pennsylvania governor and the first director of U.S. Homeland Security.