No candidate in the past 25 years has been viewed more unfavorably by Americans (Democrat, Republican and independent) than Donald Trump.
George H. W. Bush is the only one that really came close, with his unfavorable ratings spiking just before he lost his reelection in 1992. On the most-popular end of the spectrum are Bernie Sanders and Ben Carson -- both of whom are less well-known than Trump, meaning that fewer people have an opinion one way or the other.
Gallup also noted earlier this week that Ted Cruz's favorability among Republicans had plunged this month, perhaps due to the increased scrutiny he's been facing as the one-time Iowa front-runner. (Or, perhaps, for other reasons.)
There's one small consolation for Republicans worried about an unpopular Trump winning their party's nomination. The third least popular candidate in Gallup's numbers is the person currently leading the Democratic field, Hillary Clinton.