WASHINGTON (Reuters) - President Barack Obama hits the road on Monday for a Midwestern bus tour that he hopes will leave doubts over his leadership behind in Washington. But the three-day trip through Iowa, Minnesota and Illinois -- three states seen as vital to his 2012 re-election campaign -- could put him in front of voters who, polls show, are furious about political gridlock in the U.S. capital. Iowa, which launched Obama's historic journey to the White House in 2008, has recently been playing host to Republican presidential hopefuls who have aggressively slammed his record as they criss-cross the state....