The impeachment inquiry into President Donald Trump's interactions with Ukraine has heated up over the last several weeks, bringing with it a mountain of backlash from the president and his supporters.

House Democrats have heard testimony from diplomats and officials involved in Trump's mission to get Ukrainian President Volodymyr Zelensky to look into domestic political rivals, and the withholding of $400 million in military aid to Ukraine.

An anonymous whistleblower complaint alleged the Trump administration pressured Zelensky to investigate former Vice President Joe Biden and his son Hunter Biden. Trump acknowledged asking for an investigation into the Bidens. He also asked Zelensky to look into the origins of 2016 election interference, which he has claimed, without evidence, involves Ukraine.

What the inquiry has turned up so far

More:Read all the released transcripts from the closed-door testimony in the Trump impeachment inquiry

Here is how both sides defend their views on impeachment:

Democrats' case for impeachment

House Democrats, who overwhelmingly favor impeachment, argue that Trump's conduct with Ukraine was intended for personal political gain, at the detriment of the best interests of the country.

"In the course of my official duties, I have received information from multiple U.S. Government officials that the President of the United States is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election," the whistleblower wrote in the complaint. Democratic lawmakers have said testimony supports the claims made by the whistleblower.

Democrats have honed in on a phrase Trump used in his phone call with Zelensky: "I would like you to do us a favor though." He went on to ask the Ukrainian leader to look into the details of the hacking of the Democratic National Committee prior to the 2016 election that was linked back to Russia.

And, in October the White House seemed to acknowledge, then later denied again, that the delay in military aid previously approved by Congress was, in part, tied to the theory that Ukraine was involved in the hacking of the server. Democrats' claims of a quid pro quo were revitalized.

Their theory is supported, they say, by the text messages between U.S. diplomats and testimony heard over the last several days. On Tuesday, Taylor testified he was "alarmed" by the linking of aid to investigations of Biden.

Sondland also revised his testimony on Nov. 4 to say that he now recalled a conversation with Zelensky adviser Andriy Yermak.

"I now recall speaking individually with Mr. Yermak, where I said that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks," Sondland said.

This confirms the testimony of Taylor and Morrison, who recalled hearing about Sondland's conversation.

But what does that mean?:The Trump impeachment is all about an allegation of quid pro quo.

The case against impeachment

Trump and his political allies maintain that there he did nothing wrong, calling the impeachment probe a "witch hunt" and a "lynching" as Democrats persist with their investigation of potential wrongdoing.

Key arguments coming from the Trump camp revolve around the way Democrats are operating their proceedings.

So far, testimony has taken place behind closed doors. The Constitution allows Congress to determine appropriate protocol during an impeachment, but Republicans accuse Democrats of "secrecy."

Intelligence Committee Chairman Adam Schiff, a Democrat, announced Nov. 6 that the House would begin with public testimony, following a resolution vote to open up the proceedings and releases of closed-door testimony transcripts.

"The Democrats Scam goes on and on! They Do Nothing!" Trump said in a tweet, sharing a House Republican's complaint about the closed-door testimony.

In October, Republicans who are not on the committees conducting private sessions disrupted a deposition in protest.

Trump says that he should be able to cross-examine witnesses, as well as identify and question the anonymous whistleblower. He says Democrats are violating his due process rights.

“This is a partisan inquiry,” Trump campaign press secretary Kayleigh McEnany said on Fox News. “This is serious, it’s overturning an election. And we can’t read transcripts, and we can’t present evidence and we can’t have an attorney present.”

Trump has also argued that he has a duty to ask other countries to help the U.S. investigate potential corruption, and that he never suggested a quid pro quo. He says his conversation with Zelensky was "a perfect call."

Trump and his allies have made an effort to discredit certain witnesses. He called Taylor a "Never Trumper" after the diplomat testified that Trump allies told Ukraine there was a link between military aid and an investigation into the Bidens. He used the same label for Vindman, and some conservatives questioned Vindman's loyalty to the U.S. because he was born in Ukraine.

Trump has also pointed to Volker's newly released testimony, in which he said "Well, you asked what conversations did I have about that quid pro quo, et cetera. None, because I didn't know that there was a quid pro quo."

"Witch Hunt!" Trump tweeted after the transcript was released.

Quid pro quo?:How the Trump White House's messaging changed on whether there was a Ukraine quid pro quo

Contributing: Bart Jansen, Nicholas Wu