I understand why supporters of Donald Trump are resistant to the conclusion that he abused his power by asking Ukraine to investigate a political rival.

Democrats and the traditional media have been out to get Trump from before he was inaugurated.

The Russia collusion investigation was a witch hunt. There is reason to believe that it had partisan and improper origins.

After the collusion story proved a bust, there was an attempt to blow some life into the obstruction of justice charge. But that was a lead balloon with the American people.

The Ukraine accusation is understandably seen by Trump supporters as just the latest attempt by the same folks to do Trump in. Particularly with the way House Democrats initially went about launching and conducting impeachment hearings.

But the Ukraine deal is different, something even Trump supporters should acknowledge. Trump has said he wants Republicans to defend his actions in Ukraine on the merits, not just criticise the Democratic impeachment proceedings on process grounds. Here is a good faith effort to evaluate the defences of Trump’s behaviour that are being made, and explain why they are wanting.

According to the defence, as president, Trump has a legitimate interest in seeing corruption investigated. Being a rival candidate for the presidency doesn’t provide immunity from such an investigation.

So, even if there was a quid pro quo, as certainly US diplomats interacting with Ukraine believed and as was communicated to Ukrainian officials, it doesn’t matter. Asking for such an investigation into the Bidens was a legitimate exercise of Trump’s authority as president.

Now, what the Bidens did was smarmy. In that part of the world, Hunter Biden being on the board of a Ukrainian company conveyed influence with the Obama administration. That could affect events, including the course of investigations, even without Biden doing anything overtly.

But the president of the United States doesn’t have a general writ to pursue corruption everywhere on the globe. And for there to be a legitimate presidential interest in a specific investigation, corruption cannot be a general description but a specific illegal act with a tie to the United States. If Joe Biden used his office as vice president to enrich or protect his son or himself, that would violate US law. And it would be appropriate, if unusual, for a president to ask a foreign leader to cooperate with an investigation into a violation of US law.

But there is no such US investigation into the Bidens, and based on what is currently known, no basis for one to be launched. That also would be a witch hunt. Rather than a legitimate request for cooperation with an official investigation by the US government, Trump was asking a foreign country to launch an investigation to damage Biden as a candidate in 2020.

That this was a request to benefit Trump personally and politically, rather than pursue a legitimate interest of the government of the United States, is cinched by Trump asking for coordination with Rudy Giuliani, his personal attorney. Giuliani represents Trump the candidate, not Trump the president.

The defence of this takes the following construct: Trump is president of the United States. He can act through whomever he wants. If he decides to act through Giuliani, that’s his prerogative. There’s nothing wrong with that.

That, however, isn’t the constitutional order vouchsafed us by the founders. They didn’t give the president the authority to act through whomever he wants.

Article II gives the president the authority to nominate those exercising government power, subject to the approval of the Senate. It gives Congress the authority to decide which “inferior officers” the president can appoint without the approval of the Senate.

In Federalist No. 76, Alexander Hamilton emphasises that the president has the authority to nominate, but not appoint. He says that this check on the president’s ability to staff the government “will be a considerable and salutary restraint”.

And he offers an example directly on point: “It will readily be comprehended that a man who had himself the sole disposition of offices would be governed much more by his private inclinations and interests than when he was bound to submit the propriety of his choice to the discussion and determination of a different and independent body.”

One cannot be a self-proclaimed “constitutionalist” and assert that there was nothing wrong with the way Trump deployed Giuliani in this matter.

I understand that, to many Trump supporters, he is hated by all the right people. However, believing that, and even acting on it politically, shouldn’t include defending the indefensible.

Robert Robb is a columnist for The Arizona Republic where this column first appeared.