The Democrat defecting to Donald Trump's side in impeachment

Donald Trump is all but certain to be impeached, given the Democrats have a majority in the House. Photo / AP

A Democratic congressman is expected to switch party allegiance and vote against impeaching Donald Trump today joining Republican members who have rallied round the US president ahead of the historic decision.

Jeff Van Drew, elected only last year in a traditionally Republican district in New Jersey, is reportedly set to cross the aisle during voting to reject the two articles of impeachment being debated today.

Van Drew's predicted defection has horrified his congressional staff, who have resigned en masse, but won acclaim from Trump at this moment of grave political peril.

"Jeff Van Drew is very popular in our great and united Republican Party," Trump wrote on Twitter, previewing a defection not yet formalised.

"It was a tribute to him that he was able to win his heavily Republican district as a Democrat. People like that are not easily replaceable!"

Jeff Van Drew is reportedly set to cross the aisle during voting to reject the two articles of impeachment being debated today. Photo / Getty

Van Drew has not commented, but also has not denied the speculation about his defection. He faces a tough battle to hold on to his seat next year.

The news gave Trump a timely boost ahead of what could be his darkest day in office - his formal impeachment by the House of Representatives.

Congressmen will debate two articles of impeachment today, one of abuse of power and the other of obstruction of Congress.

Both relate to Trump's behaviour in the Ukraine investigations scandal.

Trump is all but certain to be impeached, given the Democrats have a majority in the House.

Impeachment does not guarantee Trump's departure from office.

That decision will be taken by the Senate, which is preparing to hold a trial on the matter in January.

The chance of removal from the White House is slim given 67 of the 100 senators would need to support the move, meaning at least 20 Republicans would have to forgo their party allegiance and vote against the president.

Trump's fury at the impeachment push, which he has characterised as a "witch hunt", appears to have shored up any doubts among Republican congressmen.

An anti-President Trump crowd gather at a rally to protest and call for his impeachment this week in New York. Photo / AP

All Republican members in the House are expected to vote against the articles of impeachment, while a handful of Democrat rebels - including Van Drew - could join them in opposing the move.

Failing to win a single Republican vote would weaken the Democrats' attempt to portray the impeachment as a necessary step to protect the country.

The vote comes just three months after an inquiry was called by the Democrats when it emerged that Trump had urged Volodymyr Zelenskiy, the Ukrainian president, to investigate Joe Biden, the former US vice-president he could face at the 2020 election, and his son Hunter, who worked for a Ukrainian gas company.

The abuse of power article accuses Trump of harming US interests for his own political benefit by holding back $400 million ($610m) in military aid from Ukraine and the prospect of a White House meeting with Zelenskiy to secure the investigations.

Democrats allege Trump ordered "without lawful cause or excuse" his officials not to give testimony to the inquiry or hand over vital documents, undercutting the probe into his own conduct. Trump and his allies have vehemently denied both claims.

Donald Trump is furious at the impeachment push, which he has characterised as a "witch hunt". Photo / AP