Democratic presidential candidate Barack Obama is offering supporters the chance to be "the first to know" his choice for vice-presidential running mate by subscribing to an email service.

Senator Obama has largely financed his campaign with contributions from the internet and with this unusual move may expand further his list of internet-based contacts and supporters.

The email, in the name of the senator's campaign manager, David Plouffe, appeared in inboxes under the headline, "Be the first to know".

"Barack Obama is about to make one of the most important decisions of this campaign — choosing a running mate," it begins. "You have helped build this movement from the bottom up, and Barack wants you to be the first to know his choice."

Both presidential candidates are expected to nominate a running mate perceived to make up for their own shortcomings.

In Senator Obama's case, his relative inexperience is seen as his main weakness. His opponent, septuagenarian John McCain, is expected to opt for a relatively youthful colleague well versed in economics, Senator McCain's admitted short suit.

Senator Obama is still facing pressure from supporters of Hillary Clinton for a "roll call" vote at the Democratic Convention later this month that would enable the more than 1600 delegates won by Senator Clinton to demonstrate their support for her, disrupting the unified image party leaders would prefer.

Some of her supporters believe her historic achievement as a female presidential candidate was wrongly diminished in comparison to Senator Obama's place as a black candidate.