Last year's Arkle winner Voy Por Ustedes produced a storming finish to win the 2007 Queen Mother Champion Chase at Cheltenham.
Thornton was enjoying his second success of the 2007 Festival
The 5-1 shot, trained by Alan King and ridden by Robert Thornton, was following up his victory in the Arkle Trophy at last year's Festival.
Leading at the last, the six-year-old galloped on strongly to beat Dempsey by a length and a half.
The favourite Well Chief fell at the second, unseating Timmy Murphy.
Newmill and Ashley Brook set a furious pace from the start and it was no surprise that there was an early casualty.
Ashley Brook soon went on and continued to lead until after the third-last, where Dempsey was still going well.
Meanwhile, Voy Por Ustedes was making relentless progress but blundered at the second-last, which could have cost him victory.
He's a much better horse when it dries up
Winning trainer Alan King
But he recovered superbly and jumped the last strongly as Ashley Brook fell at the final fence.
It was a second success at the meeting for King and Thornton following their victory with My Way De Solzen in the Arkle Trophy on the opening day.
The winner was bouncing back after unseating Thornton in the Game Spirit at Newbury last time out.
King said: "He was a bit forgotten about after his run at Newbury, which didn't go quite according to plan, but we've been thrilled with his preparation and the ground came right for him.
"He's a much better horse when it dries up so I was quite pleased when I walked the course this morning and 'Choc' (Thornton) gave him a wonderful ride.
"He got in a bit tight at the second-last, but he stays very well. I was looking forward to today and it's great for the team."
Queen Mother Champion Chase:
1. Voy Por Ustedes (Robert Thornton) 5-1
2. Dempsey (Noel Fehily) 20-1
3. River City (Tom Doyle) 33-1
10 ran. Dist: 1½, 5