"I thought it was a great opportunity for the American people to see two very different visions for the country," Romney said. "I think it was helpful to get to describe those visions. I saw the president's vision as trickle-down government, and I don't think that's what America believes in."

But despite the momentum he is likely to earn from the debate, Romney told the gathered conservative activists that the November election would still be a "close-fought battle." And he encouraged supporters to help his campaign by volunteering and voting in the coming weeks.

Romney had not confirmed his attendance at the event, and his appearance was a surprise to attendees. The Republican presidential candidate was introduced by his sons. According to the Huffington Post, Romney joked he "heard my boys were going to be here — I couldn't resist."

After the event, Romney headed to the Denver airport. The candidate was scheduled to fly to Virginia on Thursday afternoon, where he and running mate Paul RyanPaul Davis RyanGOP leaders pitch children's health funding in plan to avert shutdown Lawmakers see shutdown’s odds rising Fix what we’ve got and make Medicare right this year MORE planned to hold a rally with country music star Trace Adkins in the early evening.