(Reuters) - Illinois, the longtime home of President Barack Obama, has chosen Democrats for president in every election since 1992 and for governor since 2002. But this year Republicans, finding a weak incumbent in Democratic Governor Pat Quinn, see an opening. The Republican front-runner ahead of the March 18 primary, venture capitalist Bruce Rauner, has emerged as a deep-pocketed and potentially formidable rival. The Republican National Committee already has staff on the ground in Illinois and plans to add more this spring. Quinn eked out a victory four years ago by less than 1 percentage point and his job approval...