Sen. Chris Van Hollen (D-Md.). (Chip Somodevilla/Getty Images)

THE RESULTS of a special counsel’s investigation into Russia’s involvement in the 2016 election will not be known for some time, but one fact is well established: The regime of Vladi­mir Putin tried to sway the results of the presidential vote. Moreover, it is likely to mount similar operations in 2018 and 2020. In that sense, the most damaging aspect of President Trump’s behavior on Russia may not be his attempts to discredit the work of Robert S. Mueller III — which so far have had scant effect — but his utter disregard of the continuing threat from Moscow.

A report this month from the Democratic members of the Senate Foreign Relations Committee, headed by Benjamin L. Cardin (Md.), describes the situation with terrifying simplicity: “Never before in American history has so clear a threat to national security been so clearly ignored by a U.S. president.” Though its hacking of the Democratic National Committee, distribution of malicious propaganda on social media and efforts to penetrate state electoral computer systems have been documented by the U.S. intelligence community and prompted the Obama administration and Congress to impose sanctions, the Putin regime evidently has not been deterred.

Since the 2016 election, Russia has tried to disrupt democratic processes in many other countries. The Senate report describes operations in Britain, Germany, France, Spain, Italy and a host of Central European and Nordic nations. While many of these states have taken countermeasures to protect themselves, the Trump administration has done next to nothing — in large part because the president refuses to recognize the threat.

In taking the initiative to impose sanctions last year, Congress mandated some countermeasures, but the administration has been slow to implement them. For example, operations of the State Department’s Global Engagement Center, which was given funds to combat Kremlin disinformation campaigns, “have been stymied by the Department’s hiring freeze and unnecessarily long delays by its senior leadership in transferring authorized funds to the office,” the report says.

It’s becoming painfully clear that if the midterm elections in November are to be protected from Russian interference, action will have to come from Congress and state governments. While states need to ensure that their electoral systems are secure, the best defense, as in the Cold War, is deterrence. Mr. Putin, who according to U.S. intelligence agencies personally ordered the attack on the 2016 election, must get the message that another such assault would incur unacceptable consequences.

That’s the logic behind bipartisan legislation introduced in the Senate this week by Sens. Chris Van Hollen (D-Md.) and Marco Rubio (R-Fla.). Called the Defending Elections from Threats by Establishing Redlines, or Deter, Act, it would mandate a robust U.S. response if Russia or any other state interfered in a federal election. Sanctions would be imposed on entire sectors of an economy, including finance, energy and defense; and senior political figures and business executives would be subject to sanctions, including an asset freeze and ban on travel to the United States.

With 10 months to go before the midterms, Mr. Trump ought to be publicly guaranteeing such a response by the executive branch in the event the vote is interfered with. Because he will not do so, Congress should act quickly.