WASHINGTON (Reuters) - President Barack Obama reached out directly on Tuesday to key Republican senators negotiating a sweeping U.S. immigration overhaul and urged them to help craft a bipartisan reform bill as soon as possible. Facing criticism for not getting personally involved in the delicate process on Capitol Hill, Obama made phone calls to three U.S. senators - Marco Rubio, John McCain and Lindsey Graham - part of a "Gang of Eight" group of Republicans and Democrats working on an immigration package. ...