(CNN) President Donald Trump now acknowledges he brought up Joe Biden in a recent call with the new leader of Ukraine, but says it's no big deal.

"Everybody does it," Trump said Monday.

That blasé attitude is not reflected in Congress, where the weekend's revelations have led more Democrats to support an impeachment inquiry against Trump. The worst possible scenario, which not everybody believes, is that Trump used his office and his authority over foreign aid to stir up trouble for a leading 2020 rival.

But the political fight is over more than just whether Trump pressured his Ukrainian counterpart, President Volodymyr Zelensky, to get involved in the US presidential election by investigating Biden and his son, Hunter, over Hunter's former business interests. It's also about what must be shared with Congress after a whistleblower raised the alarm, and just how much Trump has changed what's allowed in American politics.

"I'm leading in the polls, they have no idea how they stop me," Trump told reporters Tuesday at the United Nations. "The only way they can try is through impeachment."

This is a fast moving scandal that's hard to keep track of. Here's what we know about each of the issues at hand:

Did Trump talk to Ukraine's new president about Joe Biden?

Yes. Trump initially denied it, but over the weekend admitted that when he talked to Zelensky on July 25, he brought up Joe Biden. Trump also made clear that in his mind, Biden = corruption.

On Sunday, Trump said of the conversation:

"We had a great conversation. The conversation I had was largely congratulatory, was largely corruption -- all of the corruption taking place, was largely the fact that we don't want our people like Vice President Biden and his son (adding to the corruption)," Trump told reporters.

The call is part of the whistleblower complaint against Trump. The Wall Street Journal reported Friday that Trump told Zelensky "that he should work with [Mr. Giuliani] on Biden, and that people in Washington wanted to know" whether allegations were true or not, a person familiar with the matter said, according to the Journal. While Trump pressed Zelensky, he did not bring up military aid on the call, CNN reported Friday.

Mr. Giuliani is former New York mayor and Trump lawyer Rudy Giuliani, who has been calling for months for a new inquiry into Biden's role in a corruption purge in Ukraine during the Obama administration. He renewed those calls during a remarkable interview with CNN's Chris Cuomo last Thursday during which he said he asked officials in Ukraine to investigate Biden.

Why is anyone suggesting Trump pressured Zelensky or tied a Biden investigation to military aid?

At the same time Trump was mentioning Biden to Zelensky, Giuliani was actively pushing Ukraine's top prosecutor, who has since left that position, to officially investigate Biden and his son.

That's also about the same time Trump's administration was putting the skids on nearly $400 million aid, including $250 million in military aid and $141 million from the State Department, US officials familiar with the matter tell CNN.

Sen. Chris Murphy, a Democrat from Connecticut, said on Twitter last week that after he met recently with Zelensky, there was no question the Ukrainian was concerned both about the aid being cut off and about Giuliani's calls for an investigation in Biden.

"Zelensky did not explicitly connect the two in our meeting, but he was VERY concerned about the cut off of aid, and VERY aware of the conversations that Rudy Giuliani was having with his team. I told him it was best to ignore requests from Trump's campaign operatives. He agreed," Murphy explained in a series of tweets.

Zelensky did not explicitly connect the two in our meeting, but he was VERY concerned about the cut off of aid, and VERY aware of the conversations that Rudy Giuliani was having with his team. I told him it was best to ignore requests from Trump's campaign operatives. He agreed. — Chris Murphy (@ChrisMurphyCT) September 20, 2019

An aide to Zelensky, Andriy Yermak, summed up his contacts with Giuliani in an interview published Monday.

"So, Giuliani and I talked on the phone, and after that we personally met in Madrid. I repeated to him the same points. He noted that it was important for him to hear that the new Ukrainian government would preach the principles of openness and legality, will fight corruption, and set as its goal to make Ukraine successful."

Who is Zelensky?

Zelensky is a former comedian, and like Trump, a newcomer to politics. Before his election in April, he was better known as the star of a TV show in which he played the President of Ukraine . When he won a landslide victory over Petro Poroshenko, he promised to clean up corruption.

Did Zelensky feel pressured by Trump to investigate Biden?

There are conflicting accounts on this. Ukrainian Foreign Minister Vadym Prystaiko said over the weekend that his country would not take sides in a US election. "I know what the conversation was about and I think there was no pressure," he told the Ukrainian media outlet Hromadske, according to Reuters . "This conversation was long, friendly, and it touched on many questions, sometimes requiring serious answers."

Did Trump directly tie military aid to Ukraine to investigating Biden?

We don't entirely know. It does not appear Trump ever explicitly told Zelensky that if he didn't investigate Biden, he wouldn't get the military aid. There is a transcript of the call, but it's not clear if or when anyone outside the White House will see it. Some details of the call have been reported, including by CNN

"I did not make a statement that 'you have to do this or I'm not going to give you aid.' I wouldn't do that. I wouldn't do that," Trump told reporters on Monday.

He also suggested it's his hope the call's transcript is released: "It was a very nice call. I hope you get to see it and I hope you get to see it soon."

When was the military aid to Ukraine frozen?

CNN confirmed that Trump asked his acting chief of staff Mick Mulvaney to put a hold on the millions in military aid to Ukraine roughly one week before a call with Zelensky, two senior administration officials said on Monday. Officials from the Office of Management and Budget told officials from the Pentagon and State Department about the freeze at a meeting in mid-July, according to The Washington Post

When and how were the funds frozen?

Trump admitted he withheld the money in comments to reporters at the UN Tuesday. He said he wanted Europe to do more for Ukraine. But then he added the money was ultimately paid. He didn't point out it was ultimately paid because of an objection from Democrats.

"My complaint has always been -- and I'd withhold again, and I'll continue to withhold until such time as Europe and other nations contribute to Ukraine," he said. Because they not doing it, it's the United States. We're putting up the bulk of the money.

Mulvaney has long been skeptical of the aid. Former national security adviser John Bolton and Secretary of State Mike Pompeo did not agree on much, but they did agree that Ukraine should get the aid. Congressional staffers say Congress was in the dark on the holdup.

"The administration did not notify us that the aid was frozen," according to a Senate aide. "The House Appropriations Committee learned of it on or about August 23rd and informed us, and then the first news stories appeared days later. In response to our inquiries, DoD confirmed to us that the funds were frozen on August 28th."

Further, according to CNN reporting, officials in Ukraine were never formally told the funds were being frozen.

Why does Trump keep returning to Biden and Ukraine?

Politics. Trump wants to attack Biden's credibility. Every time Trump is asked about his conversation with Zelensky, he returns to his message about Biden, as he did on Twitter Monday.

"Now the Fake News Media says I 'pressured the Ukrainian President at least 8 times during my telephone call with him.' This supposedly comes from a so-called 'whistleblower' who they say doesn't even have a first hand account of what was said. More Democrat/Crooked Media con.......Breaking News: The Ukrainian Government just said they weren't pressured at all during the 'nice' call. Sleepy Joe Biden, on the other hand, forced a tough prosecutor out from investigating his son's company by threat of not giving big dollars to Ukraine. That's the real story!"

An important note: There have been no serious allegations of specific wrongdoing by either Joe or Hunter Biden. An investigation into a Ukrainian natural gas company, Burisma Holdings, where Hunter Biden was on the board, had been shelved by a prosecutor before Biden and many other Western officials and Ukrainians called for him to step down. Biden had threatened aid in the form of loan guarantees if the prosecutor did not leave office.

When did Trump's focus on Biden and Ukraine start?

This is a story that has been percolating for some time. Giuliani has long lobbied Ukraine to investigate Biden's call in 2016 to remove the country's top prosecutor, who at one point had been investigating the natural gas company connected to Hunter Biden. CNN reported in August that Giuliani and Yermak, the aide to Zelensky, had spoken several times over the phone and met in Madrid.

Now according to The Washington Post, Giuliani said he has had five conversations with Yermak.

Separately, Giuliani told the Post he did not address the issue of US aid to Ukraine in the Madrid person to person meeting this summer. "I was not involved in the aid at all," he said, referring to military aid. "I had no idea the $250 million was on the table."

He said that Yermak was pushing for an Oval Office meeting between Trump and the Ukrainian president and that the Ukrainians were "embarrassed" there had not been one yet.

Giuliani has said he essentially stumbled upon the Biden element. He had been pushing in Ukraine on an effort to clear Paul Manafort, the former Trump campaign chairman who is currently in jail for tax evasion on money he made for political consulting in Ukraine and elsewhere.

Why does Ukraine need US military aid?

Ukraine is dealing with Russian-backed separatists in the Eastern part of the country and still opposes Russia's 2014 takeover of Crimea, which had been part of Ukraine. The country is very much standing between Vladimir Putin's Russia and the West.

What is the aid used for?

The assistance, according to previous CNN reports , is for sniper rifles, rocket-propelled grenade launchers, counter-artillery radars, night vision and military medical treatment -- all of which Ukrainians could use to counter pro-Russian separatists.

What was the official reason that aid to Ukraine was frozen?

The White House has said it wanted to make sure the new administration in Ukraine would stay true to its word and cut down on corruption.

Vice President Mike Pence, for instance, denied earlier this month that the holdup had anything to do with Biden, but he did say, "We have great concerns about issues of corruption... I mean, to invest additional taxpayer in Ukraine, the President wants to be assured that those resources are truly making their way to the kind of investments that will contribute to security and stability in Ukraine."

That's very similar to language Trump used Sunday, except Trump tied the idea of corruption in Ukraine to Biden, perhaps further suggesting a link between the money and the investigation.

Trump returned to corruption during his comments to reporters at the United Nations on Monday.

"If you don't talk about corruption, why would you give money to a country that you think is corrupt?" he said. "One of the reasons the new president got elected is he was going to stop corruption. It's very important that on occasion you speak to somebody about corruption. Very important."

How was the aid unfrozen?

There was bipartisan opposition to the freeze up on Capitol Hill. CNN reported on September 12 that the administration had suddenly dropped its opposition to the aid after Sen. Dick Durbin, an Illinois Democrat, got bipartisan support for an amendment to force the administration's hand. Sen. Lindsey Graham, a South Carolina Republican and Trump ally who nevertheless supported the aid to Ukraine, gave Durbin credit for making the aid happen.

"So why was it released? Because of your amendment, that's why it was released," Graham said during an appropriations hearing earlier in September. "Because I was going to vote for it, so I think they got the message. If you're listening in the Ukraine on C-SPAN, you're going to get the money."

Has Ukraine gotten any of the aid?

"The Department of Defense was able to begin obligating these funds last week. We will continue to leverage all available tools to expedite the implementation of these programs. We expect the majority of the funds to be obligated by the end of the fiscal year," according to Cmdr. Sean Robertson, a Pentagon spokesman.

Some $60 million had been obligated fairly quickly (as of last Monday), according to a US official. The Pentagon is confident that it can obligate $220 million before the end of the fiscal year and is working with Congress to get an extension to continue the aid delivery. The House voted last week to extend the window for the aid through November.

Why is it important when Trump talked to Zelensky?

It's completely incredible that Trump may have pressured the Ukrainian president, essentially asking him to investigate a US political rival, in the very midst of denying his campaign colluded with Russia's election interference in 2016.

On Wednesday, July 24, former special counsel Robert Mueller testified before Congress about all of the Trump campaign interactions with Russia in 2016 and explained why his report did not recommend charges against the President or his aides for seeking political help from a foreign government.

"NO COLLUSION, NO OBSTRUCTION!" Trump tweeted that day and his allies hailed the end of calls for impeachment. Turns out things were just beginning. He talked to Zelensky the next day.

What is the fight over the whistleblower?

While there had been plenty of reporting on Biden's involvement in the Ukraine, on Giuliani's quest to highlight it for campaign season and on the temporary halt of aid to Ukraine, the whistleblower complaint is what tied them all together.

But the White House tried very hard to keep it quiet. While Congress should be entitled under the law to hear about whistleblower complaints, the White House had justified keeping it from Congress. The Intelligence Community Inspector General informed lawmakers that the complaint existed, but no one outside the intelligence community or the White House has actually seen it.

That's because the DNI has, on the advice of the White House and Department of Justice, resisted providing the report to Congress and have said it is not actually covered by the whistleblower law.

What are Democrats saying?

Even some Democrats who had opposed impeachment proceedings against Trump are now saying there is no alternative.

"I don't think we have a choice under the Constitution," said Texas Rep. Vicente Gonzalez on CNN Monday. "We must move forward with impeachment proceedings."

"I don't think we'll have much of a choice," he said. "It seems like the President is really pushing us for impeachment proceedings. I don't know if he's baiting us because he has a personal agenda that he thinks this might be helpful to him."

What is Trump saying now?

He was asked exactly during an appearance Monday at the United Nations if he's taking impeachment seriously.

"Not at all seriously," Trump said, denying any wrongdoing and staying impeccably on-message that this whole thing has more to do with Biden.

"We had a perfect phone call with the president of Ukraine," Trump said. "Everybody knows it's just a Democrat witch hunt. Here we go again. They failed with Russia, they failed with recession and now they're bringing this up. The one who has the problem is Biden. Biden did what they would like to have me do except one problem, I didn't do it."

This story has been updated with additional developments Tuesday.