The biggest mystery currently looming over the political world is who Mitt Romney will pick as his running mate. The campaign has actively built suspense over the announcement — offering supporters a chance to meet the eventual pick and tweeting out the names of some contenders believed to be on a shortlist. On Tuesday, the campaign released a smartphone app called "Mitt's VP" and promised that those who download it will be the first to know who Romney chooses. Audie Cornish has more.