The New York Times reported the presumptive Democratic nominee had not yet notified his selection, but was focusing mainly on Virginia Governor Tim Kaine, Indiana Senator Evan Bayh and foreign policy expert Senator Joseph Biden.

The Times, in a report on its website, cited unnamed Obama advisers as saying he reached his decision last week while on vacation on Hawaii and the campaign was readying an elaborate rollout for his selection. Obama will first inform his grass roots network of supporters about the pick in an unprecedented email and text message blitz.

The paper cited aides as saying the announcement would come at the earliest early tomorrow, and no later than Friday.

Of the three possible picks cited by the Times, Kaine would boost Obama's hopes of winning battleground state Virginia. He has appeal with white working class Democratic voters, but lacks foreign policy experience.

Biden, chairman of the Senate Foreign Relations committee, would compensate for Obama's inexperience in national security issues and has just returned from Georgia and issued a stiff condemnation of Russia's actions in its showdown with the Tblisi government.

Bayh, popular in his home battleground state of Indiana, also has some foreign policy expertise and might be a unity pick, having been an firm ally of Obama's vanquished Democratic foe Hillary Clinton.