The dawn of the Trump presidency is turning into a real-time history show about the Constitution’s inherent system of checks and balances on any one branch’s power. The federal judiciary kicked off the lesson when it halted the president’s immigration order. Now Congress should step up to perform its oversight role.

The intelligence committees of both the Senate and the House of Representatives were already looking into how Russia interfered with the 2016 election by hacking into email of prominent Democrats. Meanwhile the FBI is probing any involvement Trump campaign operatives Carter Page, Paul Manafort and Roger Stone may have had in the release of the emails.

But Senate Majority Leader Mitch McConnell and especially Speaker Paul Ryan are inexplicably dragging their heels about expanding their probe of former National Security Adviser Michael Flynn and the storm of suspicion he created about the White House’s relationship with Russia. Until Congress openly and thoroughly probes Flynn’s conversations with the Kremlin before Trump’s inauguration, the nation cannot have confidence that a foreign power hasn’t compromised national security.

Flynn walked the plank late Monday, 17 days after the White House was warned by the Justice Department that Flynn, a top Trump aide, was vulnerable to blackmail because of misleading statements he had given. The warning said intercepts of a call Flynn made in December to Russian Ambassador Sergey Kislyak hours after the Obama administration expelled dozens of Russian spies, including some who lived in an Upper Brookville compound, indicated that Flynn had discussed whether Trump would reverse those decisions.

Despite that warning, Flynn still had access to intelligence data and continued to work closely with Trump. Only on Thursday did Vice President Mike Pence learn of the warning and that Flynn had misled him about the nature of the call, he said. Despite weeks of controversy about what happened, Flynn stayed on the job until The Washington Post revealed that the White House had been warned earlier.

This disturbing timeline creates questions that only Flynn and other witnesses can answer under oath. Was Flynn freelancing in his call to the ambassador or doing it with the president-elect’s approval? The day after that call, Russian President Vladimir Putin stunningly announced he would not retaliate for those harsh sanctions and instead invited the children of U.S. diplomats to a Christmas party at the Kremlin. In response, the president-elect tweeted of Putin, “I always knew he was very smart.”

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Why did the president and the White House counsel wait so long to inform Pence, who was the one who denied reports that Flynn sought to undermine the U.S. government? Does Russia have any other information that compromises this administration?

Meanwhile, Putin is testing Trump, who is hardening his stance on Russia after facing increasing political pushback over Flynn’s efforts to play nice and reduce harsh economic sanctions imposed over Russia’s aggression in Ukraine. Putin has moved a midrange ballistic missile system closer to Europe, in possible violation of a Cold War treaty, and last week Russian military jets buzzed a Navy destroyer in the Black Sea.

Until Republicans probe Flynn’s dealings with Russia, they risk being accused of putting their party’s interests ahead of those of the nation.

— The editorial board