Factbox: Possible 2016 White House field shifts as Romney exits

(Reuters) - The calculus for the 2016 race for the White House has shifted as conservatives reposition themselves after 2012 Republican presidential nominee Mitt Romney's decision not to run again. The following are a few of the possible contenders: THE REPUBLICANS JEB BUSH Is the country ready for a third Bush president? The former Florida governor, brother of one president and son of another, has been testing the waters of a White House bid. His Right to Rise political action committee hired a