MADISON, Wis. (AP) — Warning the GOP's presidential race has become too nasty, Scott Walker suspended his 2016 campaign on Monday and called on some of his Republican rivals to do the same, citing an urgent need to "clear the field" to help defeat GOP front-runner Donald Trump. "Today, I believe that I am being called to lead by helping to clear the field in this race so that a positive conservative message can rise to the top of the field. With that in mind, I will suspend my campaign immediately," Walker said in a Madison press conference.