Donald Trump described himself on Wednesday as the worst-treated president ever after Democrats announced a formal impeachment inquiry against him.

House Speaker Nancy Pelosi said in announcing the probe on Tuesday afternoon that Trump had betrayed his oath of office by seeking help from Ukraine to hurt his Democratic rival Joe Biden.

In the latest of a blast of tweets in response to that bombshell announcement, Trump said: “There has been no President in the history of our Country who has been treated so badly as I have.

“The Democrats are frozen with hatred and fear. They get nothing done. This should never be allowed to happen to another President. Witch Hunt!,” Trump tweeted.

The dramatic move marked the first step in a complex process that stands little chance of driving Trump from office. And it pushed US politics into a perilous new chapter 14 months before new elections for control of the White House and Congress.

Pelosi, the speaker of the House of Representatives, and other Democratic Party leaders had resisted taking the step for months, preferring to focus on the coming election fight.

But a combination of the most recent allegations that Trump offered Ukraine aid in return for help to damage Democratic White House frontrunner Biden, and a groundswell of support for impeachment among the party's rank and file, appeared to tip the balance.