U.S. President Donald Trump blasted the “impeachment crusade” against him in a strongly-worded letter to a top Democrat on Tuesday, ahead of a key vote on whether or not to kick him out of the White House.

A six-page letter to House of Representatives Speaker Nancy Pelosi rails against the Democrat-led effort to impeach Trump for pressuring Ukraine to dig up dirt over his political rival, former Vice President Joe Biden. “I write to express my strongest and most powerful protest against the partisan impeachment crusade being pursued by the Democrats,” wrote Trump, a Republican. “This impeachment represents an unprecedented and unconstitutional abuse of power by Democrat Lawmakers, unequaled in nearly two and a half centuries of American legislative history.”

In the signed missive, Trump said that he did nothing wrong in seeking foreign investigation of political rivals and added voters would punish Democrats in the 2020 presidential vote laboring on an irrelevant issue.

“By proceeding with your invalid impeachment, you are violating your oaths of office, you are breaking your allegiance to the Constitution, and you are declaring open war on American Democracy,” wrote Trump.

“You dare to invoke the Founding Fathers in pursuit of this election-nullification scheme — yet your spiteful actions display unfettered contempt for America's founding and your egregious conduct threatens to destroy that which our Founders pledged their very lives to build.”

The letter was published as House Democrats and Republicans rowed over the rules of debate over whether to impeach Trump, which is expected to lead to votes that will make him the third president to be impeached in American history. The Democrat-controlled House is expected on Wednesday to vote to impeach Trump for abusing his presidential powers by pushing Ukraine to help himself politically and then obstructing Congress by blocking the subsequent investigation.

No Republicans are expected to vote to impeach Trump. This sets the stage for a trial of Trump in the Senate. There, a two-thirds vote of members is needed to eject Trump from the Oval Office, meaning the Republican majority in the upper house can easily block the Democrats.

The Democratic-led House of Representatives launched impeachment proceedings against Trump on Sept. 24 following claims by a whistle-blower that the commander-in-chief had sought to pressure Ukraine to interfere in the 2020 US presidential elections.

In a July 25 phone call, Trump allegedly made $391m military aid to help Ukraine fight Russian-backed separatists and a coveted White House meeting for Ukrainian President Volodymyr Zelensky contingent on a “quid pro quo” arrangement. In return, Zelensky was supposed to open corruption probes into Joe Biden, a former US Vice President, and his son, Hunter, who served on the board of Ukrainian gas company Burisma, as well as into alleged Ukrainian meddling in the 2016 U.S. election.

The elder Biden is a leading candidate in the race to win the Democratic nomination and challenge Trump in 2020. Trump, a Republican, has accused Democrats of time-wasting and says the inquiry amounts to a “witch hunt”.