Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE warned that Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE should "drop out" if she can't win Tuesday's Kentucky primary as he laid into his likely general election opponent in a series of tweets.

"Do you think Crooked Hillary will finally close the deal? If she can't win Kentucky, she should drop out of the race," Trump tweeted Tuesday night before adding "system rigged."

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He's repeatedly chided Clinton for Sanders' performance in the Democratic primary, questioning how he could have wrapped up his nomination before her considering she came in as a heavy favorite.

And he's railed on the Democratic primary as unfair for its reliance in part on superdelegates, party leaders who have the freedom to influence the process, accusing the party of trying to keep Sanders down.