Howard dean, Rudy Giuliani, Steve Forbes, and Hillary Clinton
Dean especially never really broke out of the low 20s, and Forbes didn't hold the lead for a significant chunk of 1995 in the 96 race. Giuliani is a fair point, but unlike Trump, had no support in Iowa or New Hampshire. Trump is currently narrowly leading IA and massively leading NH. As for Clinton, like pointed out before, that was a 2 1/2 candidate race. Clinton came in third in Iowa with 30% of the vote...the winner of Iowa in the GOP 2016 primary will be lucky to hit that number.
Think about it this way. Bernie Sanders could lose Iowa by 30 points, 65-35 (due to the 15% threshold at each station, O'Malley won't even register), and Sanders' humiliating defeat would still be a higher vote share than the winner of the GOP contest in Iowa. That's the difference between a two way race and a 14 way race.
It's also worth pointing out that in contrast, Trump is still polling 10-15 points higher than
Romney was at this point, when that was a
6-person race.
Trump's stability in the polls looks remarkably similar to Romney's throughout 2011-2012, except that Trump is polling even
higher than Romney in an even bigger clusterf[inks].
