The determination reportedly reached by the CIA that Russia intervened in the presidential election to help sway it to Donald Trump is stunning and alarming. Trump’s dismissal of that finding as “ridiculous” is no less troublesome.

Trump might well have won without Russian help in the form of emails hacked from Democratic officials and released by WikiLeaks. His economic message resonated more strongly than Hillary Clinton’s. But this is not about election results. It’s about foreign intervention in our election. Trump needs to stop acting like a candidate and start acting like the president he is going to be after the Electoral College makes the election results official next Monday.

This is not a threat to him. It’s a threat to the nation.

Sen. Mitch McConnell, the Republican majority leader, and Sen. Chuck Schumer, the future Democratic minority leader, head a bipartisan list of congressional members demanding an official investigation, which can be done by the Senate Intelligence Committee. Americans need to know what, if anything, a rival and aggressive superpower did to discredit Clinton in an effort to swing the election results to a candidate it preferred. As McConnell put it, “We need to approach all these on the assumption the Russians do not wish us well.” Sen. John McCain, who has little doubt there was Russian hacking, called it “another form of warfare.”

The investigation must deal with the extent of Russian involvement and the intent of that nation’s meddling. The FBI is less sure about the CIA’s conclusion — why? These are questions that must be answered credibly and as soon as possible. Trump’s recent remarks and actions suggest he still does not understand the danger posed by Russian President Vladimir Putin. Trump says he wants a better relationship with Putin, a thug and enemy of the West. Now Trump’s apparent front-runner for secretary of state appears to be ExxonMobil CEO Rex Tillerson, a friend of Putin who has complained that U.S. sanctions against Russia have made doing business there more difficult. It’s a poor choice.

But Trump’s underestimation of Russia is not the only worrisome thing here. His trashing of the CIA is yet another attack on an essential American institution. The CIA has made mistakes and it needs to be closely watched by the president, but publicly trashing it is unprecedented. And it’s possibly damaging to its work around the world.

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Eventually, Trump will need the intelligence community. If he undermines the nation’s faith in these agencies, he undermines faith in his own decision-making. Similarly, saying as he did on Fox News on Sunday that he is smart to skip daily intelligence briefings and leave them to his generals and Vice President-elect Mike Pence is wrong. The military does not run our country, civilians do. And he must know what he doesn’t know about the dangers in the world.

Trump doesn’t want the legitimacy of his election eroded. But he is eroding his own credibility with his stand on Russia. The nation deserves to know what happened. The president should want that, too. — The editorial board