• Video report by ITV News Washington Correspondent Robert Moore

Trench warfare is now the only business in this town. In fact, on Thursday the trenches were further fortified. The bunkers were reinforced. Weapons are being loaded. Impeachment won’t be fought on open ground but from the narrow confines of party loyalties.

That is to say, one of the great spectacles of American democracy is underway.

Speaker of the House Nancy Pelosi called a vote to authorise an investigation into Donald Trump. Credit: AP

For the third time in modern American history, Congress is engaged in impeachment proceedings and the House of Representatives has now voted to legitimise - and accelerate - the whole process.

Republicans have complained that Democrats are staging a coup with secret hearings intended to bring down a president. To counter that accusation, the Speaker of the House Nancy Pelosi called a vote to authorise the investigation.

The resolution was passed along party lines - 232 to 196 - with Democrats saying that American democracy and the constitution are at stake.

Republican leaders claim that the investigation is a “cult” and that Democrats are using Soviet-style tactics. Let’s just say, this is not a bipartisan affair.

What the vote really revealed is that there will be no mass crossing of no-man’s-land to defect to the other side. This is no Watergate, when even Republicans realised that President Nixon had to be evicted from office for the sake of American democracy.

This time, Republicans leaders and their core voters are sticking with Trump through thick and thin. His Senate firewall is intact, his House loyalists are digging in, and the President has been real-time tweeting from the White House, now calling this (predictably) the greatest witch-hunt in American history. Yes, impeachment has overtaken the Mueller investigation to win that status.

Former top national security adviser to President Donald Trump, Tim Morrison, arrives for a closed door meeting to testify as part of the House impeachment inquiry. Credit: AP

So although the House will almost certainly vote to impeach the President by Christmas time, there is no appetite by the Republican-controlled Senate to convict him at the trial-stage of the process.

It requires a two-thirds majority in any case, so the possibility that Trump will be removed from office is remote.

But impeachment shouldn’t be seen as just a ritual. In the eyes of Democrats, it is the business of Congress to hold the President to account, and that is precisely what they intend to do.