How much have Trump’s dealings with Ukraine deviated from the presidential norm?

In the House impeachment inquiry Tuesday, the National Security Council’s top Ukraine expert, Lt. Col. Alexander S. Vindman, is expected to testify that he was disturbed when President Trump asked Ukrainian President Volodymyr Zelenskiy to “investigate” one of Trump’s political rivals. Vindman was one of the officials on the July 25 phone call that kicked off the impeachment inquiry. Vindman’s testimony, along with the expected testimony this week of the NSC’s Tim Morrison, will be the first time the House will hear from currently serving White House officials.

But while the NSC officials’ testimony may add to the growing picture, it will build on last week’s testimony from William B. Taylor Jr., acting ambassador to Ukraine, whose written testimony already altered our view of what happened.

Taylor’s opening statement stood out for several reasons. He outlined in careful, chronological detail that “there appeared to be two channels of U.S. policy-making and implementation, one regular and one highly irregular.”

His statement showed him to be a seasoned diplomat who kept detailed records and respected process and communication within the executive branch. And those qualities revealed just how much Trump’s foreign policy deviated from the norm. Separate channels have long been part of U.S. foreign policy, and leaders regularly consider domestic politics in their foreign policy plans. But Trump’s particular use of his separate channel, and this kind of quid pro quo, has not.

Domestic politics are a normal part of foreign policy. Inviting foreign interference in U.S. elections is not.

It certainly isn’t news to international relations scholars that democratic leaders consider domestic politics when making foreign policy.

U.S. leaders often discuss domestic politics in their internal policy debates. For example, Lyndon B. Johnson frequently and candidly discussed making policy in Vietnam with one eye on the 1964 election. In many phone calls with aides, Johnson fretted over the need to placate Henry Cabot Lodge Jr., the former Republican senator and potential future presidential candidate then serving as ambassador to South Vietnam. In a May 1964 phone call with Robert McNamara, Johnson urged his secretary of defense to back Lodge’s recommendations, saying that if they did what Lodge suggested, “we’re not in too bad a condition politically,” but otherwise, “we are caught with our britches down.”

Leaders have also talked about U.S. domestic politics with foreign leaders. For instance, Bill Clinton explained to Russian President Boris Yeltsin in May 1995 that adding countries like Poland to NATO would solidify the support he received from American voters of Central and Eastern European descent in his reelection bid, saying: “Wisconsin, Illinois and Ohio are key; they represented a big part of my majority last time — states where I won by a narrow margin. The Republicans think they can take away those states by playing on the idea of NATO expansion.” But Clinton wasn’t asking Yeltsin for any favors, adding: “Let me be clear, Boris: I’m not bargaining with you. I’m not saying, ‘Do what I want or I’ll change my position.’ ”

Political scientist Paul Poast explained that leaders commonly deal in some foreign policy behaviors we might call quid pro quos, such as side payments or issue linkages, i.e., trading policy concessions or linking progress on one issue to another.

But Trump asking a foreign leader for help investigating a political rival crossed the line into using secret government communications and relations for personal gain. The resulting whistleblower complaint and political pressure pushed Trump to release a rough transcript of his July 25 phone call with Zelenskiy, setting off the impeachment inquiry.

Separate channels aren’t new. But this one was highly unusual.

Taylor’s testimony clarified the picture of a separate channel for Ukraine policy that evaded the normal diplomatic and bureaucratic processes. Separate channels aren’t new, although the historical record shows some familiar patterns.

Sometimes, presidents rely on a loyal aide or insider and cut other officials out of the loop. For example, Richard Nixon and Henry Kissinger frequently circumvented the State Department when making foreign policy — and even went behind the back of their own negotiator in talks leading up to the 1972 SALT I accords.

Presidents also use special messengers for added credibility. During the Cuban missile crisis, John F. Kennedy relied heavily on his brother Robert, who was his attorney general, not his secretary of state, using him to make sure the Soviets knew when messages came directly from the president.

Nor is Trump the first president to contact foreign leaders through aides who don’t have official roles at all. After being defeated by Franklin Roosevelt in the 1940 presidential election, Wendell Willkie asked FDR about his close adviser Harry Hopkins, who lacked a formal White House position but became a key go-between with British Prime Minister Winston Churchill and Soviet leader Joseph Stalin. Roosevelt replied: “Someday you may well be sitting here where I am now as president of the United States. And when you are … you’ll learn what a lonely job this is, and you’ll discover the need for somebody like Harry Hopkins who asks for nothing except to serve you.”

But while these messengers may have been loyalists or insiders, the message was still official U.S. national security policy.

Another way presidents have set up separate channels is through special envoys. Such envoys are typically experienced and knowledgeable enough to gather information and focus intensively on an issue, ideally with clear White House backing. And it’s true the Trump administration appointed longtime diplomat Kurt Volker to serve as special envoy to Ukraine (albeit part time and unpaid).

But the channel became highly irregular through the involvement of individuals like former New York mayor Rudolph W. Giuliani and Republican Party donor turned ambassador to the European Union Gordon Sondland, who had no experience in dealing with a geopolitically important and sensitive issue like Ukraine. The contrast between Sondland and the highly experienced Taylor was very clear in what is publicly known of their testimony, as the latter kept detailed notes and could draw on decades of experience in explaining what was and wasn’t proper.

It’s still the official channel that matters

In this case, what may matter in the end is something that has plenty of precedent: the regular diplomatic channel. By acting as a diplomat normally does — keeping records, attempting to stick to the formal process, communicating with his colleagues in Washington and, ultimately, testifying to Congress, which authorized the aid in the first place — Taylor took the regular, usually invisible work of diplomacy and used it to show us how Trump crossed a line.

After all, lines are only useful if we can see when they are crossed.