Trump tweets with fury, says impeachment push helps him

Fifty-eight floors above Manhattan, President Donald Trump watched his legacy change and his political future grow more uncertain.

The president, back in his hometown of New York for the UN General Assembly, was taking "executive time" at his Trump Tower penthouse late Tuesday afternoon when Speaker Nancy Pelosi announced that the House was launching a formal impeachment inquiry against him.

Pelosi's move increases the odds that Trump will become the third US president to be impeached.

It was a step more than 2 years in the making and one that moves the president farther down the path of self-styled political martyrdom.

The product of Trump's norm-breaking presidency and Democrats' lingering anger over the outcome of the 2016 election, the impeachment inquiry has largely been welcomed by the Republican president's advisers, who believe it could backfire against Democrats.

The president himself said the move could help his electoral chances, but he reacted in the moment with a cascade of angry tweets that accused Democrats of engaging in "a witch hunt" and "presidential harassment".

A short time earlier, as word of Pelosi's decision first emerged, an agitated Trump sized up the politics of the moment and the developments that have quickly enveloped his presidency since it was revealed that a whistleblower complaint accused him of pressuring the leader of Ukraine to dig up damaging material about political foe Joe Biden's family.

"They're going to lose the election, and they figure this is a thing to do," Trump told reporters.

Speaking of Pelosi, he added, "If she does that, they all say that's a positive for me, for the election. You could also say, 'Who needs it? It's bad for the country.'"