By MICHAEL R. CRITTENDEN And NEIL KING JR. Republican presidential candidate Rick Santorum handily won Missouri's primary election Tuesday night and was showing signs of strength in Minnesota's caucuses, threatening to deal national front-runner Mitt Romney a setback in two Midwestern states. With results from Colorado just trickling in, the returns from the other two states holding nominating contests Tuesday looked to give a jolt of momentum to Mr. Santorum, a former Pennsylvania senator.