What is Trump trying to hide?

We know President Trump and his campaign either colluded with the Russian effort to undermine U.S. democracy or tried mightily to do so. We know Trump has apparently obstructed justice to try to halt investigation into what happened. What we don’t know is whether Congress will do its sworn duty to protect the Constitution.

We also don’t know what else special counsel Robert Mueller might have discovered, especially about the Trump family’s international financial dealings. Or what Mueller might be learning from Trump’s former national security adviser, Michael Flynn, who pleaded guilty Friday to lying to the FBI and is cooperating with investigators. Or how far Trump, who is increasingly frantic, might yet go to squash the Mueller probe.

It is true that there is no federal statute against “collusion.” But a specific law is not necessary for citizens and their representatives in Congress to make a judgment: Is it acceptable for a presidential candidate and his campaign to encourage an adversarial foreign power’s efforts to meddle in the U.S. election process — and then seek to reward that foreign power by easing sanctions? Yes or no?

I’m no fan of conspiracy theories, which usually fall apart under scrutiny; and I’m not interested in carrying water for the Democratic Party, which should have been able to beat Trump, who was manifestly unqualified and unfit, no matter what the Russians did. But what we have learned thus far is truly shocking.

In July 2016, Trump issued a public plea: “I will tell you this, Russia: If you’re listening, I hope you’re able to find the 30,000 emails that are missing.” He was referring to deleted material from the private email server Hillary Clinton used when she was secretary of state. Previously, according to U.S. intelligence officials, state-sponsored Russian hackers had obtained thousands of private emails from the Democratic National Committee and Clinton campaign chairman John Podesta, and the material was being released in a manner clearly intended to damage the Clinton campaign.

We now know that in June 2016, three of the most important figures in the Trump campaign — Trump’s son, Donald Trump Jr.; his son-in-law, Jared Kushner; and his campaign chairman, Paul Manafort — had eagerly met with a Kremlin-tied lawyer who promised to share damaging information the Russian government had on Clinton. We also know that in April 2016, another go-between had promised Trump foreign policy adviser George Papadopoulos “thousands of emails” containing “dirt” on Clinton.

Did anyone report these shady approaches to the FBI? No.

We know that in the last days of the campaign, Russian cyberwarriors targeted the social media accounts of potential Trump voters in key states. We don’t yet know how they aimed their propaganda so accurately.

We have learned, however, that after the election, the Trump transition team actively undermined sanctions that President Obama had imposed on Russia for its election interference. Flynn discussed relaxing the sanctions with Russian ambassador Sergey Kislyak — then lied about it to the FBI. Transition adviser KT McFarland wrote in an email that the sanctions would make it harder for Trump to improve relations with Russia, “which has just thrown the U.S.A. election to him.” The White House says McFarland meant only that Democrats would claim Russia had helped Trump win.

And we know that members of the Trump campaign’s inner circle consistently failed to disclose their meetings with Russian officials and emissaries. There is a pattern of behavior here. It may or may not be illegal, but it is certainly shocking and unacceptable.

As for obstruction of justice, Trump tweeted Saturday that “I had to fire General Flynn because he lied to the Vice President and the FBI.” This indicates the president knew Flynn had committed a crime before meeting with FBI Director James Comey and, according to Comey, asking him to drop the Flynn investigation. Trump later fired Comey, telling NBC’s Lester Holt that the reason was the Russia probe. Trump has reportedly hectored other administration officials and members of Congress to stop investigating the Russia connection.

One of Trump’s lawyers, John Dowd, claimed that he, not Trump, authored that incriminating tweet. He later argued to Axios’ Mike Allen that the president “cannot obstruct justice because he is the chief law enforcement officer” under the Constitution.

With Flynn now cooperating, Mueller’s investigation enters a new phase. But let’s not lose sight of the big picture. Ask yourself a common-sense question: If nothing wrong happened with Russia during the campaign, why is Trump so desperate to cover it up?

Eugene Robinson is syndicated by The Washington Post Writers Group.