A "landslide victory" means winning by a very large margin or by a huge amount. It suggests overwhelming success or winning by a wide difference. Full definition
McKenna, Magarelli, and Wilber, finishing in that order, won landslide victories in their bids for reelection. (hudsonvalleyone.com)
Opinion polls originally predicted a huge landslide victory for the Conservative Party and May aimed to substantially increase her party's slim majority. (en.wikipedia.org)
And while de Blasio's landslide victory over Republican Joe Lhota in November was seen as a break with the last 12 years of Mayor Michael Bloomberg's administration, the incumbent is leaving office with generally high marks, the poll found. (nystateofpolitics.com)