Story highlights Sen. Sanders' supporters said hacked DNC emails showed the party favored Clinton in the presidential primary

Former Clinton campaign manager Robby Mook, a CNN political commentator, said Trump should address threats to US elections

Trump's tweet comes as Congress and special counsel Robert Mueller probe Russia's efforts to influence the US election

(CNN) President Donald Trump lashed out at Hillary Clinton and the Democratic Party in a tweet Sunday morning, saying the former secretary of state "colluded" with the party in the presidential primary to defeat Sen. Bernie Sanders.

"Is she allowed to so collude?" the President asked. "Unfair to Bernie!"

Hillary Clinton colluded with the Democratic Party in order to beat Crazy Bernie Sanders. Is she allowed to so collude? Unfair to Bernie! — Donald J. Trump (@realDonaldTrump) June 25, 2017

The tweet is an apparent reference to a cache of nearly 20,000 hacked Democratic National Committee emails released by WikiLeaks nearly one year ago that Sanders' supporters' said bolstered their claims that the party favored Clinton.

The tweet comes as congressional committees and Department of Justice special counsel Robert Mueller continue investigations into Russia's efforts to influence the US election and the Trump campaign's contacts with Russian officials, including allegations of collusion between his campaign and Russia in an effort to defeat Clinton during the presidential campaign. Trump, his campaign and Russian officials have denied any collusion took place between them.

Without mentioning Sanders' name, one of the hacked DNC emails appeared to show a DNC staffer asking others to ask the candidate about his faith, saying it would weaken him in the eyes of Southern voters. Another seemed to depict an attorney for Clinton's campaign advising the committee on how to defend her against an accusation by the Sanders campaign of not living up to a joint fundraising agreement.

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