Donald Trump has done “far worse than anything Nixon did”, a leader of the impeachment process in the House said on Sunday, adding: “The question is why are Republicans placing this president above their oath of office.”

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Nixon resigned in 1974, before he could be impeached over the Watergate break-in and his efforts to cover it up.

Trump faces two articles of impeachment related to pressure he applied on Ukraine to launch political investigations that would help his bid for re-election as well as obstruction of Congress by blocking the appearance of key witnesses before congressional hearings.

On ABC’s This Week on Sunday, Adam Schiff, chair of the House intelligence committee, said: “If anything this president’s conduct is far worse than anything Nixon did, far more sweeping in its obstruction of accountability, far more damaging to our national security and the cover-up that was Watergate.”

A full vote of the House of Representatives is expected on Wednesday. Should the articles pass by simple majority – as expected despite opposition from swing-seat Democrats including Jeff Van Drew of New Jersey, expected to switch to the Republicans over the issue – Trump will become only the third president to be impeached. Andrew Johnson, in 1868, and Bill Clinton, in 1999, survived trials in the Senate.

Trump is set to face trial in January. The Senate majority leader, Mitch McConnell, has said the president will not be convicted and added that he is coordinating closely with the White House on strategy.

Democrats have protested but on Saturday the South Carolina Republican senator and Trump ally Lindsey Graham stoked their wrath further, telling a CNN interviewer in Doha he was “not trying to be a fair juror here” and that “this thing will come to the Senate and it will die quickly, and I will do everything I can to make it die quickly.”

On Sunday, key Republicans followed Graham’s trail of breadcrumbs. The Kentucky Republican senator Rand Paul predicted on CNN’s State of the Union his party would vote as a bloc in the Senate to acquit Trump, without even defections from critics of the president, such as Mitt Romney of Utah, or electorally vulnerable moderates such as Susan Collins of Maine.

Asked how he could square his predetermination of the outcome of the trial with his role as an impartial juror as laid out in the US constitution, Paul said: “This is a very partisan thing. This is a disagreement, people on the Democratic side don’t like President Trump so they decided to criminalise politics.”

Paul claimed making up his mind before the trial was consistent with his constitutional duties. Other Republican senators have gone further in expressing their disdain for the impeachment process. On Sunday, many insisted the spotlight should instead be on former vice-president Joe Biden and his son Hunter, whose business interests in Ukraine are at the heart of Trump’s moves in that country.

Hunter Biden had a board position at a Ukrainian energy company while his father was vice-president and pushing for anti-corruption reforms in Kyiv. There is no evidence of wrongdoing by him or his father but Graham, appearing on CBS’s Face the Nation, insisted: “These are legitimate concerns about what happened in the Ukraine. I love Joe Biden, but none of us are above scrutiny.”

Schiff accused Graham and his party of an “excess of extreme partisanship”, adding: “It is more important to one party that the president of their party remain in office than what he does to the country and that I think puts us deeply at risk.”

Jerry Nadler, chair of the judiciary committee which drew up the articles of impeachment after Schiff’s intelligence panel staged hearings, told ABC Trump posed “a continuing threat to the integrity of our elections now”.

Asked why impeachment mattered if the outcome in the Senate was a foregone conclusion, he said: “This is not a one-off. Impeachment is not a punishment for past behaviour.

“This president sought foreign interference in the 2016 election, he is openly seeking foreign interference in the 2020 election and he poses a continuing threat to our national security and the integrity of our elections to our democratic system itself.

“We cannot permit that to continue.”

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The Delaware senator Chris Coons echoed him, telling NBC’s Meet the Press that “if the Senate Republican majority refuses to discipline [Trump] through impeachment, he will be unbounded. And I’m gravely concerned about what else he might do between now and the 2020 election, when there are no restrictions on his behaviour.”

Speaking to ABC, the Texas senator Ted Cruz, a bitter foe of Trump during the 2016 primary who is now a dutiful supporter, was correspondingly unconcerned.

“This is trying to undermine an election,” he said. “And it’s why it’s not going to go anywhere in the Senate.”

Public opinion remains split. According to a CBS News poll released on Sunday, 42% of respondents said Trump should be convicted and removed from office while the same number said he should either not be convicted or that no Senate trial should be held.