President Trump is once again claiming his campaign was illegally spied on, and this time, he's bringing Sen. Bernie Sanders into the fray.

In an early morning tweet, the president said: "Looking more and more like the Trump Campaign for President was illegally being spied upon (surveillance) for the political gain of Crooked Hillary Clinton and the D-N-C. Ask her how that worked out - She did better with Crazy Bernie. Republicans must get tough now. An illegal Scam!"

Meanwhile, Sen. Bernie Sanders, on "Face the Nation" Sunday morning, said that the president's performance after the Helsinki summit calls into question whether he understands Russia's actions during the election.

"Either he doesn't understand it, or perhaps he is being blackmailed by Russia because they may have compromising information about him or perhaps also, you have a president who really does have strong authoritarian tendencies and maybe he admires the kind of government that Putin is running in Russia," Sanders said.