A U.S. military veteran and oyster farmer on Tuesday launched a bid to unseat Republican U.S. Senator Susan Collins in Maine, as his party fights an uphill battle to try to recapture control of the chamber in next year's midterm elections.

Graham Platner, a 40-year-old Army and Marine veteran who served four combat tours in Iraq and Afghanistan, launched his campaign for the Senate in Maine on Tuesday, joining a growing Democratic primary field seeking to take on Republican Sen.