When Volodymyr Zelensky won Ukraine’s presidential election in July, President Donald Trump gave him a congratulatory phone call. It now seems, however, that a friendly congratulation might not have been his only motive. During the call, Trump asked Ukraine to investigate Joe Biden and his son for allegedly corrupt business dealings in Ukraine. Last week, an anonymous whistleblower from a U.S. intelligence agency released a public statement implicating Trump for using U.S. foreign policy to further his own political career, and Trump was forced to release the call’s transcript.

What has been set off is arguably the greatest scandal of Trump’s presidency so far. Top House Democrats have re-energized their calls for an impeachment hearing, and the public seems to have their back. According to Politico/Morning Consult’s poll, for the first time, more of the American public supports initiating impeachment proceedings than not.

But what does impeachment actually entail? Why does Trump’s phone call alone constitute grounds for impeachment?  

The impeachment process was set into the constitution as a check against the executive branch of government. Article One Section Five of the Constitution gives the House of Representatives the power to impeach the president for “high crimes and misdemeanors,” meaning actions that demonstrate a president’s disloyalty or inability to act as the chief executive of the United States. The Senate has the power to try all impeachments. If an impeachment trial were to initiate, it would require at least ⅔ of the Senate to approve it for the impeachment to become binding.

A key concern of the founding fathers was that the United States could go under tyrannical foreign rule again if democratically-elected leaders held conflicts of interests regarding foreign countries. For this reason, the Constitution specifically prohibits gifts, payments or other things of value from leaders of foreign countries. By asking a foreign leader to dig up dirt on a major political opponent, Trump has entered the grey area of whether such a request constitutes a high crime. Speaking to Zelensky, Trump asked for a “favor” from Ukraine, saying “our country has been through a lot and Ukraine knows a lot about it.”

Ukraine has long been a target for Russia because of supposed cultural ties. For centuries, Russian leaders have referred to Ukraine as “Little Russia” because of their shared linguistic heritage. Russia justifies its bid to expand world influence by casting the liberal democratic system of the European Union as corrupt and exploitative, a charge which Trump repeats. Russia views retaking Ukraine as part of its manifest destiny, the idea of a Russian-led world in which Russia, not the West, holds sovereignty over all its cousin cultures. For the most part, in the past, Ukrainians stood against Russian interference in their politics. The annexation of Crimea sparked massive protests that were largely ignored by the European Union’s leadership despite many of the protestors waving EU flags.

Trump is using this grievance as an opportunity to attempt to reposition the U.S., not Europe, as the reliable world superpower to guard against tyranny. Zelensky and Trump expressed their willingness to increase economic and security cooperation between both countries. Zelensky said he is willing to buy more weapons from the United States. Trump lambasted the European Union for failing to adequately protect Ukraine, and in his characteristic brand of flattery, emphasized what good people his Ukrainian friends are.

Advocates of impeachment alleged that by taking advantage of the U.S.’ strategic partnership with Ukraine to demolish a political opponent, he is acting in violation of the Emolument Clause and committing an unacceptable abuse of power. Opponents of impeachment, mostly Republicans, argue that since there was no explicit quid pro quo, there are no grounds for impeachment.

In the meantime, Trump has taken to the defensive, suggesting that the impeachment hearing is a power grab simply to get him out of office.

In an ideal world, impeachment hearings should be non-political, but support for impeachment inevitably splits along party lines, especially with today’s level of political polarization. However, there still exists the possibility that impeaching Trump could backfire by making him a martyr. Legally, it is possible to impose a lifetime ban on running for office against impeached presidents, but with a Republican-controlled Senate, it is highly unlikely that the House would even be able to remove Trump from office.