John McCain's campaign manager has told Fox News that the Republican presidential candidate has chosen his running mate, who is expected to be revealed in the coming hours.

Rick Davis confirmed that after months of vetting and discussions Mr McCain's search for his Republican vice-presidential nominee is over. It is widely expected that the two will appear together at a rally today in Dayton, Ohio, just hours after Barack Obama accepts the Democratic nomination before a crowd of 75,000 in Denver at 3.30am GMT.

The identity of the choice is still known to only a handful of Mr McCain's closest aides. Speculation has focused on Mitt Romney, the former Massachusetts governor and onetime presidential rival, Tim Pawlenty, the Minnesota governor, Joe Lieberman, Al Gore's 2000 running mate, and even Meg Whitman, the former head of eBay and a McCain adviser.

Meanwhile, Mr McCain is set to release a prime-time campaign advertisement at the same time as Mr Obama will make his speech. For once, Mr McCain will not attack his rival, but praise his achievement in becoming the first black presidential nominee.

Mr Obama is speaking on the 45th anniversary of Martin Luther King's "I have a dream" speech.

Mr McCain will say in the commercial: “How perfect that your nomination would come on this historic day. Tomorrow, we’ll be back at it. But tonight Senator, job well done.”

Times Recommends

• Experts baffled by vanishing sea lions
• CIA caught in a dirty and secretive war
• Double life of Umar Farouk Abdulmutallab