WASHINGTON (AP) — The Republican candidates for president will gather Wednesday for their third debate amid fresh volatility in an already chaotic race, with Ben Carson surging past Donald Trump in Iowa and one-time front-runner Jeb Bush under pressure to prove he's still a viable candidate for the GOP nomination. The soft-spoken Carson has been a low-key presence in the first two GOP debates, but the retired neurosurgeon is likely to get more attention from moderators — as well as his fellow candidates — after a series of preference polls show him atop the field in Iowa. Slower-than-expected fundraising has led Bush to slash spending and overhaul his campaign structure, and he's voiced frustration with the way the unusual race has progressed. Taken together, it's a Republican field that remains crowded and unwieldy three months before the lead-off Iowa caucuses and New Hampshire primary. The political rookies appealing to voter anger with Washington have ceded no ground and establishment politicians are still waiting for the race to turn their way — and increasingly wondering if it ever will. Trump remains the dominant force, commanding media attention, drawing large crowds and leading in most early voting states.