U.S. presidential hopeful Donald Trump's support among Republicans has dropped 12 points in less than a week, marking the real estate mogul's biggest decline since he vaulted to the top of the field in July, according to a Reuters/Ipsos poll.Trump was the favorite of 31 percent of Republicans in a rolling poll in the five days ended on Nov. 27. That was down from a peak of 43 percent registered on Nov. 22.