Arms sales and foreign aid have long been a part of the United States’ carrot-and-stick approach to foreign policy. But President Donald Trump’s call with the Ukrainian leader is something entirely different.

Making access to American weapons or dollars contingent upon behaviors that support U.S. interests is standard procedure.

In that regard, Trump’s withholding of nearly $400 million in military aid for Ukraine for much of the summer — the money was released on Sept. 11 — almost seems like a routine practice, business as usual in the give-and-take of international negotiations.

What makes this case unique, as we learn more about Trump’s communications with Ukrainian President Volodymyr Zelenskiy, is the ask itself.

[Crime or ‘high crime?’ Trump’s Ukraine call spurs legal debate]

During the phone call, a read-out of which the White House released this week, Zelenskiy indicated that Ukraine was eager to buy Javelin anti-tank missiles produced by U.S. defense giants Raytheon Co. and Lockheed Martin Corp., which would help defend against Russian heavy armor units.

Trump replied, “I would like you to do us a favor, though,” and then proceeds to ask the Ukrainian president to open an investigation into former Vice President Joe Biden, a top contender for the Democratic presidential nomination in 2020.

“There’s a fine tradition of using foreign aid dollars to try to promote or deter behaviors — but because those behaviors are perceived as helpful or harmful to U.S. interests, that is, to the safety and well-being of Americans,” said Heather Hurlburt, director of the New Models of Policy Change project at the center-left think tank New America. “This case is not at all relevant to that calculus. No one in the administration has alleged that the aid was being withheld in order to get Ukraine to do or not do something.”

Foreign aid is intended to help promote U.S. security, democracy and prosperity, and/or to meet the needs of others when their humanitarian circumstances collide with American values about the basic safety and rights every person should have, she added.

“It’s not a slush fund for presidential campaigns,” Hurlburt said.

Richard Fontaine, CEO of the Center for a New American Security and former foreign policy adviser to the late Arizona Republican Sen. John McCain, said it is not unusual for a president to connect foreign or military aid to a specific policy outcome that is aligned with the national interest.

“Part of this turns on the president’s motives,” he said.

Multiple administrations have sought to reduce the scale of corruption in the Ukrainian government, so that it could be a better partner to the West and a better ally against Russia, he says. Trump has said he was also interested in corruption as a general matter, Fontaine noted.

“If that were the case, it wouldn’t be terribly exceptional,” said Fontaine, a former Senate Foreign Relations Committee and National Security Council staffer. “The suspicion is that that is not [Trump’s] actual motive, but to solicit an investigation of a domestic political opponent.”

[Trump walked fine line on quid-pro-quo threat during call with Ukraine leader]

The U.S. has used restrictions on foreign aid before to compel certain kinds of behaviors, usually in service of a broader foreign policy goal, said Mieke Eoyang, vice president of the National Security Program at Third Way, another center-left group.

For example, under amendments to the laws governing how the State and Defense departments distribute foreign aid known as the Leahy Law after sponsor Vermont Democratic Sen. Patrick J. Leahy, the United States is prohibited from funding countries that commit gross violations of human rights.

“None of that is targeted at an individual or for the benefit of an individual,” Eoyang said.

On the Hill, the appropriateness of Trump’s handling of the aid to Ukraine is largely seen through a partisan lens.

To Wisconsin GOP Sen. Ron Johnson, the president asking for a favor is merely a reflection of the president’s speech patterns, not a demand.

“You’ve heard President Trump talk, that’s President Trump,” said Johnson, chairman of the Homeland Security Committee and a member of the Foreign Relations panel. “Where has he made this contingent on anything?”

Sen. Lindsey Graham, a Trump ally who is a member of Foreign Relations, disputed any suggestion of quid pro quo.

“You can’t withhold foreign aid for personal reasons, and that didn’t happen,” the South Carolina Republican said. 

[Opinion: Trump’s smoking-gun summary]

Virginia Democratic Sen. Tim Kaine, who sits on the Armed Services and Foreign Relations committees, disagreed, saying the July 25 conversation represented a clear case of quid pro quo.

When Congress raised the issue of Saudi Arabia’s record on human rights, Trump insisted that arms sales go through, that they support good American jobs, Kaine said. But when Zelenskiy mentions an interest in buying U.S.-made missiles, Trump shifts the conversation to investigating Biden.

“He should be celebrating that Ukraine wants to pay America money for Javelin missiles and we are allies against Russia. Instead, he immediately switches over to the investigation,” Kaine said.

Sen. Christopher S. Murphy, a member of the Foreign Relations Committee, said it is appropriate to use foreign aid as a lever to try to effectuate U.S. government aims, but not for a president’s reelection efforts.

“There’s no fuzzy line here,” the Connecticut Democrat said. “You can use foreign aid to try to press a country to do something if it’s in the national security interests of the United States. You can’t use foreign aid to try to bully a country into interfering in an American election.”

Pennsylvania Rep. Chrissy Houlahan was one of the seven Democratic freshmen with national security backgrounds who published an op-ed in The Washington Post on Monday calling for the House to launch an impeachment inquiry.

Trump wasn’t using the funds as leverage to advocate a U.S. policy, said Houlahan, a former Air Force officer who is a member of the House Armed Services and Foreign Affairs committees.

“This is a personal agenda, a personal ask that he’s using U.S. taxpayer dollars that had been appropriated though the proper channels to ask for something that would politically benefit him,” she said. “It’s totally different.”