Four contenders:

Mitt Romney Former Massachusetts governor who lost out to Mr McCain in a bitter primary race but has since rallied aggressively on behalf of Mr McCain.

Tom Ridge The first secretary of homeland security under President George W. Bush and is also a former governor of Pennsylvania: a potentially decisive battleground.

Tim Pawlenty
Current Minnesota Governor Tim Pawlenty, who will be in Denver on Thursday to help undermine the Democrats, is well-liked by conservatives for his anti-abortion stance and evangelical background.

Joseph Lieberman
A dark horse in the running and close friend of Mr McCain. The former Democrat was Al Gore's running mate in 2000 and then had a falling out with his party over the war in Iraq.