Improve the bracketed part of the sentence.
Donald Trump (congratulated Putin on) landslide election victory.
"Congratulated Putin on" is correct here.
The preposition on is used with congratulate when expressing one’s good wishes in the context of a happy event.
Example: Congratulations on your marriage Or Congratulations on your win
The preposition for is used with congratulate when praising them for an achievement.
Example: Congratulations for winning the lottery or Congratulations for saving the kids.