Democratic lawmakers' case for impeaching President Donald Trump grew stronger on Tuesday when the top US diplomat in Ukraine testified that the White House withheld assistance to Kiev "for domestic political reasons".Bill Taylor told lawmakers that Washington's envoy to the European Union repeatedly said Trump demanded his Ukrainian counterpart announce an investigation into 2020 presidential contender Joe Biden.READ | 'Anonymous' Trump critic coming out with bookTaylor's explosive testimony - which was leaked to US media - added to the allegations that Trump illegally pressured Ukraine to help boost his political prospects in the US.The mounting evidence could support multiple impeachment charges that Trump has abused the power of his office, violated US election laws and illegally obstructed a congressional investigation.Here is what we know:The scandal blew up in September when an explosive whistleblower complaint came to light.READ | Trump blasts inquiry as 'lynching' as key witness testifiesThe complaint alleged that Trump pressured Ukraine's President Volodymyr Zelensky in a July 25 phone call to investigate the Bidens, as well as claims that Kiev helped Democrats in the 2016 election, in exchange for the release of aid frozen a week earlier.Biden's son had served on the board of a powerful Ukraine energy company, Burisma, and Biden himself was involved in Ukraine policy when he was vice president.By trying to involve Ukraine in US politics, the whistleblower expressed concern that Trump "had clearly committed a criminal act".According to the whistleblower and testimony from Taylor and others, Trump had begun pressuring Zelensky to investigate the two issues soon after the Ukrainian leader's election in April.The White House was leveraging a meeting Zelensky had requested with Trump and $400m in military aid that Kiev needed to fight Russian-backed rebels.Taylor said the demands were transmitted by Trump's personal lawyer Rudy Giuliani and some other US diplomats operating an "irregular" channel to UkraineAid frozen before callSix days before the July call, according to the whistleblower and Taylor, Trump's chief of staff Mick Mulvaney had the military aid frozen.In the call, according to a White House memorandum, Trump responded to Zelensky's request for the aid by saying: "I would like you to do us a favour though."Trump then mentioned both the 2016 issue and the Bidens, and said: "A lot of people want to find out about that."He then concluded: "So, if you can, look into it."Text messages and phone calls between US diplomats in Giuliani's backchannel support the allegation that the White House linked military aid to Kiev opening investigations.A week before the call, Kurt Volker, the special representative on Ukraine, texted Gordon Sondland, the US ambassador to the European Union: "Most impt is for Zelensky to say that he will help investigation."On August 13, Volker drafted a statement for Zelensky to issue that read: "We intend to initiate and complete a transparent and unbiased investigation of all available facts and episodes, including those involving Burisma and the 2016 US elections."And Taylor testified that Sondland told him on September 1 that Trump "wants President Zelensky to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 US election."Aide admits 'quid pro quo'The White House has refused House demands to provide documents related to the Ukraine dealings, and Trump has denied linking aid to political help.But the nine officials who have testified have all reportedly supported the allegations against the president.Two have said that Trump's then-national security advisor John Bolton was alarmed by the efforts to pressure Ukraine and branded it a surreptitious "drug deal" to be avoided.And last week, Mulvaney flippantly told reporters that there was indeed a quid pro quo - the aid for the investigations."That's why we held up the money," he said. "Get over it."What next?Adam Schiff, the House Intelligence Committee chair leading the impeachment inquiry, said last week that there are "a great many" interviews to come, but lawmakers are aware of the need to move quickly.Once finished, they will release the interview transcripts and then draw up articles of impeachment - formal legal charges against the president.That could happen before the end of the year. The Democratic-controlled House is then expected to quickly vote to approve the articles.Trump would then stand trial in the Senate, but convicting him could be difficult because it would require two-thirds of the Republican-controlled body.