Barreling through last-minute procedural hurdles thrown up by Republicans and mewls of complaint from Donald Trump, the House of Representatives impeachment juggernaut bore down on its goal Wednesday, with the impeachment of the US president likely sometime after nightfall.

Calling Trump “an ongoing threat to our national security and the integrity of our elections”, the House speaker, Nancy Pelosi, opened the debate just after midday on two articles of impeachment, charging Trump with abuse of power and obstruction of Congress. A simple majority vote in favor of either article would result in Trump’s impeachment.

Trump was not planning to stick around for the big moment. An accident of scheduling had him heading to a campaign rally in a small city in Michigan Wednesday evening, setting up another split-screen moment in a country hamstrung by separate political realities.

“Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG!” Trump said in an early morning tweet. “A terrible Thing.” Later Trump tweeted, in all caps, “This is an assault on America.”

But Trump’s fierce efforts to seal himself in a bubble of adulation, and his insistence that Republicans pretend away the existence of facts threatening to pierce that bubble, could not divert the reality coalescing on Capitol Hill.

Wearing a large pin of the ceremonial mace of the chamber as she rose to speak on the House floor, Pelosi warned that “our founder’s vision of a republic is under threat by actions from the White House”.

“It is tragic that the president’s reckless actions make impeachment necessary,” she said. “He gave us no choice.” Praising the “moral courage of our members”, some of whom court political risks by backing impeachment, Pelosi announced six hours of debate on the matter. Her speech was met with sustained applause from her caucus.

Republican Doug Collins of Georgia, the ranking member on the judiciary committee, replied that impeachment had always been an “inevitability” under the Democrats. “The founders were very concerned about a partisan impeachment in which the majority in their strength can do whatever they want to do regardless of the facts,” he said.

After a three-month investigation, members of Congress were prepared to act against Trump for his scheme to cheat in the 2020 election, as Democrats charge, by pressuring Ukraine to manufacture bad news about the former vice-president Joe Biden, one of Trump’s rivals, and then blocking congressional oversight.

In a marathon and at times painfully repetitive partisan volley, members exchanged one- and two-minute blocks of speaking time. Republicans accused Democrats of having a vendetta against Trump and running what they said was a corrupt process, while Democrats taunted Republicans for not even attempting to defend Trump on the merits.

“They cannot articulate a real defense of the president’s actions,” said Jerry Nadler, the judiciary committee chair.

Trump was set to become the third president in US history to be impeached, joining Andrew Johnson in 1868 and Bill Clinton in 1998. Richard Nixon resigned in 1974 before he could be impeached.

Following his impeachment, Trump would face a congressional trial in the new year in the Republican-controlled Senate, where a two-thirds majority would be required to convict and remove Trump from office. With no Republican senators currently voicing support for impeachment, Trump appeared safe to survive.

Zero Republican representatives, likewise, had signed on in advance of Wednesday’s vote, to the House impeachment case, fueling the party’s charges that the proceedings were driven by partisanship. Democrats replied that Republicans were hostage to Trump and unable to deliver a sound judgment in the matter.

Two Democrats, including one whose staff said he would soon be switching parties to the Republican side, broke with their party on an early procedural vote. With four vacant seats in the House, Democrats, who hold a 233-seat majority, would need 216 votes to impeach Trump. They repeatedly sailed over that number in a series of votes to overcome procedural roadblocks thrown up by Republicans as the Wednesday session got under way.

The House minority leader, Kevin McCarthy of California, presented a resolution seeking to condemn Adam Schiff, the chair of the intelligence committee, and Nadler for what McCarthy said were abuses of power and violations of procedure. The resolution was tabled without a vote.

Schiff, whose committee performed the bulk of the investigative work in the impeachment inquiry, took to the floor just before 4pm to detail the case against Trump.

“The president of the United States was willing to sacrifice our national security by withholding support for a critical strategic partner at war in order to improve his re-election prospects. But for the courage of someone willing to blow the whistle, he would have gotten away with it.

“Instead, he got caught. He tried to cheat and he got caught.”

Trump was scheduled to attend what was billed as a Merry Christmas rally in Battle Creek, Michigan, a shrinking food manufacturing center in a county that swung from supporting Barack Obama in 2012 to backing Trump in 2016.