US House of Representatives Speaker Nancy Pelosi called US President Donald Trump an ongoing threat to American democracy as the bitterly divided chamber engaged in an impeachment debate before historic votes on two charges accusing the Republican president of abusing his power and obstructing Congress.

The Democratic-controlled House launched a planned six hours of debate on the two articles of impeachment - formal charges - arising from the president�s actions toward Ukraine, with time equally divided between the parties and no amendments.

Wednesday�s votes set the stage for a trial in the Republican-led Senate in January.

No president has ever been removed from office via the impeachment process set out in the Constitution, and Republican senators have given little indication of changing that.

As the debate unfolded, Trump on Twitter called the Democratic-led proceedings AN ASSAULT ON AMERICA and on his party.

Trump would become only the third US president to be impeached.

In a series of speeches on the House floor, Republicans defended the president and accused Democrats of seeking to topple him from power using an unfair and rigged process to nullify the results of the 2016 election.

Meanwhile, the Republican chairman of the US Senate Judiciary Committee announced that he would want any impeachment trial in the Senate to be quick and without new witnesses.

When it gets here, my goal is to have as short a trial as possible, Senator Lindsey Graham told reporters. I am not going to support witnesses being called for by the president. I am not going to support witnesses being called for by (Democratic leader) Senator Schumer.

Graham told reporters that the process in the House was beginning to weaponize the use of impeachment for partisan purposes.