It will be disastrous for Democratic presidential candidate Barack Obama if he chooses Hillary Clinton as his running mate, the former chairman of the US Republican National Committee has said.

"That would be disastrous for him because it would destroy the whole idea of change of the Democratic campaign," said Frank Fahrenkopf, who was chairman of the Republican Party from 1983 to 1989 during the Reagan administration.

The choice for running mates is regarded as one of the most important decisions Obama and Republican candidate John McCain will take in the next five months before the elections.