US Republican presidential candidates Ted Cruz and John Kasich told reporters they were feeling optimistic, as voters headed to the polls in four states on Tuesday."I am encouraged that more and more we are seeing Republicans uniting behind our campaign," Cruz, a senator from Texas, told reporters in Raleigh, North Carolina."Donald [Trump] is upset, and I will predict he will engage in more attacks. They will be more personal, they will be more nasty, and we're going to continue not to respond in kind," he added of the party's current front-runner.Kasich also struck a positive note while campaigning in Lansing, Michigan."This is what we always planned was to head north. You know, we didn't set the schedule, but I think the schedule is moving our way," the Ohio governor told reporters after a rally.