Donald Trump was at greater risk of impeachment than at any time in his presidency on Tuesday, after reports that he apparently sought to blackmail a foreign government by withholding military aid unless it helped investigate a political opponent.

Even as the president took centre stage at the United Nations in New York to venerate his nationalist ideology, Democrats met behind closed doors on Capitol Hill in Washington to decide whether the time had come to seek his removal from office.

Trump ordered White House staff to withhold nearly $400m in aid to Ukraine days before he pressured the country’s president to investigate Joe Biden, the former US vice-president and frontrunner for the Democratic presidential nomination in 2020, according to the Washington Post.

On Tuesday afternoon, Biden, in an address delivered from Wilmington, Delaware, the state he represented in the Senate, said that Congress would have “no choice but to initiate impeachment” if the president refused to comply with requests from Congress for information about dealings with Ukraine. It was further than the former vice-president has gone before in supporting impeachment calls.

“I can take the political attacks. They’ll come and they’ll go and in time they’ll soon be forgotten,” Biden said. “But if we allow a president to get away with shredding the constitution, that will last forever.”

The scandal appeared to cut through with greater force and clarity than the drawn-out saga of his alleged collusion with Russia during the 2016 election. There was a sense of gathering momentum as more Democrats came out in favour of impeachment.

Q&A How do you impeach the US president? Show Article 1 of the United States constitution gives the House of Representatives the sole power to initiate impeachment and the Senate the sole power to try impeachments of the president. A president can be impeached if they are judged to have committed "treason, bribery, or other high crimes and misdemeanors" – although the US Constitution does not specify what “high crimes and misdemeanors” are.

The formal process starts with the House of Representatives passing articles of impeachment, the equivalent of congressional charges. According to arcane Senate rules, after the House notifies the Senate that impeachment managers have been selected, the secretary of the Senate, Julie Adams, tells the House that the Senate is ready to receive the articles. Then impeachment managers appear before the Senate to “exhibit” the articles, and the Senate confirms it will consider the case. The presiding officer of the Senate notifies the supreme court chief justice, John Roberts, of the impending trial. Roberts arrives in the Senate to administer an oath to members. The presiding officer will then administer this oath to senators: “I solemnly swear that in all things appertaining to the trial of the impeachment of Donald Trump, now pending, I will do impartial justice according to the constitution and laws, so help me God.” The Senate must vote on a resolution laying out ground rules for the trial including who the key players will be, how long they will get to present their cases and other matters. After the Senate is “organized”, the rules decree, “a writ of summons shall issue to the person impeached, reciting said articles, and notifying him to appear before the Senate upon a day and at a place to be fixed by the Senate”. A president has never appeared at his own impeachment trial. Trump will be represented by the White House counsel, Pat Cipollone, and his personal lawyer Jay Sekulow, among others. After the oath, the trial proper will begin. Senators may not speak during the proceedings but may submit written questions. The question of witnesses and other matters would be decided on the fly by majority vote. A time limit for the proceedings will be established in the initial Senate vote. The senators will then deliberate on the case. In the past this has happened behind closed doors and out of public view. The senators vote separately on the two articles of impeachment – the first charging Trump with abuse of power, the second charging him with obstruction of Congress. A two-thirds majority of present senators – 67 ayes if everyone votes – on either article would be enough to convict Trump and remove him from office. But that would require about 20 Republicans defections and is unlikely. The more likely outcome is a Trump acquittal, at which point the process is concluded. Two presidents have previously been impeached, Bill Clinton in 1998, and Andrew Johnson in 1868, though neither was removed from office as a result. Richard Nixon resigned in 1974 before there was a formal vote to impeach him. Tom McCarthy in New York

“I truly believe the time to begin impeachment proceedings against this president has come,” John Lewis, a hero of the civil rights movement and moral authority in the party, told the House of Representatives. “To delay or to do otherwise would betray the foundation of our democracy.”

There was also support from an influential group of seven first-term members of Congress with military or national security backgrounds, who said that “if true”, the president’s actions would “represent an impeachable offense”.

Meanwhile, Dick Durbin, the second-ranking Democrat in the Senate, who forced the White House to eventually release the aid to Ukraine, told reporters the time to act was now. The Connecticut senators Richard Blumenthal and Chris Murphy backed impeachment. Media reports said Biden himself would announce his support for impeaching Trump if the White House did not cooperate with Congress’s whistleblower investigation.

After months of procrastination among Democratic leaders, some believe that impeachment is now inevitable, setting the stage for a vicious 2020 election.

I truly believe the time to begin impeachment proceedings against this president has come John Lewis

All eyes were on Nancy Pelosi, the House speaker, who has repeatedly said an impeachment inquiry should be bipartisan and supported by the American people. Pelosi still lacks support from Republicans and a majority of the people. She was due to hold meetings with key committee chairs. It was reported that she would then announce a formal impeachment inquiry.

Trump scorned the idea of impeachment, as it overshadowed his typically bellicose “America first” UN address. He told reporters: “I think it’s ridiculous. It’s a witch-hunt. I’m leading in the polls. They have no idea how they stop me, the only way they can try is through impeachment … It’s nonsense …when you see the readout of the call, which I assume you’ll see at some point, you’ll understand. That call was perfect.”

He later said that he would release the “fully declassified and un-redacted transcript” of the call on Wednesday.

Impeachment has only been enforced against two previous presidents, most recently Bill Clinton. Like Clinton, Trump would almost certainly not be convicted by the Senate, which is controlled by his party, and so would remain in power. Richard Nixon resigned before impeachment could be carried out. In the 1860s, Andrew Johnson survived a Senate trial.

Q&A What is the Trump-Ukraine scandal at the heart of impeachment? Show In a July 2019 phone call, Trump asked Ukrainian president, Volodymyr Zelenskiy, to mount an investigation of his potential rival for the White House in the 2020 election, Joe Biden, and son Hunter Biden – and also to investigate a conspiracy theory that Ukraine, instead of Russia, was behind foreign tampering in the 2016 election. Trump framed the requests as a “favor” after he reminded his counterpart that “the United States has been very, very good to Ukraine”. Overshadowing the conversation was the fact that Trump had recently suspended hundreds of millions of dollars in military aid that Congress had approved for Ukraine to defend itself against Russia . News of the call emerged in a Washington Post report on 18 September that an internal whistleblower complaint, filed in August, involved “communications between Trump and a foreign leader”. Trump’s attempted dealings in Ukraine caused a scandal in US diplomatic ranks. The Democrats have obtained text messages between top US envoys in Ukraine establishing that diplomats told Zelenskiy that a White House visit to meet Trump was dependent on him making a public statement vowing to investigate Hunter Biden’s company. Trump does not dispute public accounts of what he said in the call, as established by the whistleblower’s complaint, released on 26 September, and a call summary released by the White House itself. But Trump and allies have argued that the conversation – “I’ve given you that, now I need this” – was not actually as transactional as it appears to be. Trump’s personal lawyer, Rudy Giuliani, has pressured Ukraine to smear Biden, and the whistleblower said White House officials had caused records of Trump’s Ukraine call to be moved into a specially restricted computer system. The vice-president, Mike Pence, has acknowledged contacts with Ukrainian officials while claiming to have no knowledge of Trump’s Biden agenda.

On 3 October 2019, Trump even suggested that: “China should start an investigation into the Bidens.” Unlike when he was a candidate, Trump’s invitations for foreign powers to attack his domestic political opponents now have all the power of the White House behind them. Critics say this is a plain abuse of that power and it undermines US national security because it places Trump’s personal agenda first. The Trump administration also stands accused of obstruction of Congress for resisting congressional subpoenas for documents and testimony relating to the crisis. A lot of people – from the whistleblower, to career government officials swept up in the affair, to legal scholars, to Democrats and even some Republicans – believe it’s plausible that the president has committed an impeachable offense. Tom McCarthy in New York

The Ukraine scandal came to light last week when an intelligence community whistleblower filed a report after becoming alarmed at Trump’s behaviour. The White House is refusing to release the substance of the complaint.

According to the Post, Trump ordered acting chief of staff Mick Mulvaney to suspend a near $400m payment “at least a week” before a 25 July call with Volodymyr Zelenskiy in which Trump reportedly asked the Ukrainian leader to investigate Biden and his son Hunter, who was once on the board of a Ukrainian gas company.

The company was investigated for corruption. As vice-president, Biden was part of successful international efforts to have Ukraine’s chief prosecutor dismissed.

There is no evidence of corruption in the Ukraine by either Joe or Hunter Biden.

On Tuesday, Trump confirmed that he withheld military aid – but claimed he did so over concerns the US was contributing more than its fair share compared with other countries.

Joe Biden and his son are corrupt, but the fake news doesn’t want to report it because they’re Democrats Donald Trump

A day earlier, he attempted to turn the tables and say the true corruption lay with Biden and the media.

“Joe Biden and his son are corrupt, but the fake news doesn’t want to report it because they’re Democrats,” Trump said. “If a Republican ever did what Joe Biden did … they’d be getting the electric chair by right.”

Most congressional Republicans have either defended the president or remained silent.

Democratic chairs of three House committees have demanded the White House turn over all documents related to “efforts to influence a foreign country to interfere with our upcoming election”.

Trump is due to meet Zelenskiy at the UN on Wednesday.