Barack Obama has appointed Jim Johnson, the former chief executive of Fannie Mae (NYSE: FNM - News) , the federal mortgage giant, to head a secret committee to produce a shortlist for his vice-presidential running mate, according to leaks from senior Democratic officials.

The Obama campaign, which many believe is coming under strong pressure from Hillary Clinton to take her on as his running mate, on Thursday would neither confirm nor deny the existence of Mr Johnson's committee. A campaign spokesman said: "We are not commenting on this process."

Obama officials are treading carefully for fear of offending Mrs Clinton before the primary process concludes on June 3. There are another three nominating contests left to go in Puerto Rico, Montana and South Dakota.

Mrs Clinton, who may well achieve a virtual tie with Mr Obama in the popular vote if the votes from Florida and Michigan are added but come well short of his tally among elected delegates, has reacted angrily whenever the Obama campaign has shown signs of claiming the nomination.

In addition to Mrs Clinton, who Mr Obama has said would be "on anyone's short list", Mr Johnson's committee is expected to look at potential running mates who might address his perceived weakness with certain types of voters - particularly among blue-collar whites.

Names that have surfaced include Jim Webb, the former secretary of the navy, Vietnam veteran and senator for Virginia; Tim Kaine, the governor of Virginia; John Edwards, the former vice-presidential candidate in 2004; Ted Strickland, the governor of Ohio; and Kathleen Sebelius, the governor of Kansas.

The committee would also be expected to look at figures with a military background to address Mr Obama's perceived vulnerability on national security - particularly given the strong credentials of John McCain, the Republican nominee. Names that have arisen include Sam Nunn, the former chairman of the Senate armed services committee; and Wesley Clark, the former head of Nato in Europe, who is also a backer of Mrs Clinton.

This weekend Mr McCain will also conduct an informal session with potential Republican running mates at his ranch in Arizona. Among the guests are Mitt Romney, the former rival for the nomination; Bobby Jindal, the young governor of Louisiana; and Charlie Crist, the governor of Florida. Given his age, which will be 72 in November, Mr McCain's choice is seen as particularly important.