When the story of the probable impeachment and perhaps removal of President Donald Trump is written, Thursday, October 3, is going to be one of the big days. It started off with the President publicly asking China and Ukraine to dig up, or perhaps manufacture, dirt on former Vice-President Joe Biden.Then, in the evening, bombshell after bombshell after bombshell landed, capped off late at night with House Democrats releasing text messages that detailed how the US pressured Ukraine to go after Biden and to pursue bizarre conspiracy theories about the 2016 election.Put it all together, and one thing is clear: the normal defences that a president’s allies in Congress would mount in such a situation increasingly are not available.One typical excuse made when a president gets into trouble is that there’s insufficient proof. That’s what President Richard Nixon’s defenders often resorted to, and it’s something Republicans tried out over the past week; noting that the whistleblower in the Ukraine scandal had access only to second-hand evidence.That always seemed like a weak defence, especially once the White House published a summary of a call between Trump and the president of Ukraine that corroborated the whistleblower’s account. But after Thursday, it’ll be hard to use that one at all, at least in good faith.Another classic defence is to question whether the president was personally involved. That’s how Republicans defended President Ronald Reagan during the Iran-Contra scandal, with some success. But it was never especially viable this time, and after Trump’s public performance on Thursday, it’s hopeless.That leaves the defence that Democrats successfully used for Bill Clinton: that the president’s misconduct doesn’t merit impeachment. Unfortunately for Republicans, that one isn’t credible either.Not only does asking (or pressuring) foreign nations to interfere in US elections obviously fit within traditional conceptions of “high crimes and misdemeanours”, but there’s plenty of other evidence of Trump abusing his power, obstructing justice and more. If this doesn’t merit impeachment, nothing will.And so, with reasonable arguments increasingly untenable, we’re starting to see some preposterous ones.The one to watch out for now is that impeachment and removal is inherently undemocratic. Trump won the 2016 election fair and square; preventing him from serving out his term defies the will of the electorate. This one won’t fly either — even if we ignore the Framers’ intentions and more than 200 years of precedent (including Clinton’s impeachment).The US simply isn’t a majoritarian democracy, with the president as an elective monarch who fully embodies the intent of the people.It’s a system of separated institutions sharing powers. Congress has just as much claim to being the voice of the people as the president does.And if upholding the law and respecting constitutional procedures is essential to maintaining a republic, then impeachment is an appropriate democratic remedy for malfeasance in office, especially when the offences involved are central to the operation of constitutional democracy. What all this adds up to is that there are few good-faith defences left.That doesn’t mean Trump’s allies can’t spout cheap slogans and phoney “facts” and otherwise refuse to acknowledge what’s actually going on. In fact, it’s still more likely than not that most Republicans in Congress will do exactly that. But they’re going to get undermined again and again by the facts of this case, and there’s a good chance that only the most intensely loyal party voters will be swayed by what they’re saying. Jonathan Bernstein is a Bloomberg Opinion columnist