Congressional interference into presidential conversations with foreign leaders would violate Article II. According to Mr. Trump’s critics, the inspector general for the intelligence community can forward any whistle-blower complaint to Congress if it involves a matter of “urgent concern.”

But Congress cannot subject the president to the supervision, control or review of a subordinate officer. As the Supreme Court made clear in a 1926 case, all executive branch officials exist to assist the president in the performance of his constitutional duties. An intelligence officer cannot file a whistle-blower complaint against the president, because the president is not a member of the intelligence community; nor does a presidential phone call with a foreign leader qualify as an intelligence operation. The intelligence community works for the president, not the other way around.

Under the Constitution and long practice, the president alone conducts foreign relations. As Justice George Sutherland wrote for the majority in a 1936 Supreme Court opinion (quoting Chief Justice John Marshall), the president “is the sole organ of the nation in its external relations, and its sole representative with foreign nations.”

Beginning with George Washington’s 1796 refusal to provide the House with the Jay Treaty negotiating record, presidents have claimed the right not just to communicate with foreign leaders but also to keep national security information secret. Thomas Jefferson even expanded executive privilege to protect national security information against the courts in 1807, when he refused to testify in the treason trial of Aaron Burr (Chief Justice Marshall, presiding as trial judge, accepted Jefferson’s claim).

Here, good constitutional structure matches good policy. If Congress could regulate presidential discussions with foreign leaders, presidents and foreign leaders would speak less candidly or stop making the calls altogether. United States foreign policy — approved by the American people at each election — would be crippled.

Congress would seize the upper hand in foreign affairs, which has produced disasters such as the War of 1812 and restrictions on aid to the Allies before American entry into World War II. In the 1970s, congressional interference after Watergate handicapped the efforts of Gerald Ford and Jimmy Carter to respond to the Soviet military buildup, Communist expansion in Latin America and Africa, the fall of Vietnam and Iran’s revolution. Only with Ronald Reagan’s restoration of executive power could the United States carry out the strategy that ultimately won the Cold War.

Democrats may regret again wounding the presidency when Mr. Trump’s successors grapple with the rise of China as a global power, Russia’s revanchism, Iran’s quest for regional hegemony and North Korea’s nuclear proliferation.