(CNN) With a series of damning closed-door hearings behind them and public hearings into the possible impeachment of President Donald Trump set to start later this week, Republicans have fixated on a very strange idea: We have to know who the whistleblower is!

"I consider any impeachment in the House that doesn't allow us to know who the whistleblower is to be invalid because without the whistleblower complaint we wouldn't be talking about any of this," said South Carolina Sen. Lindsey Graham (R) on Fox News on Sunday.

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That echoes what Trump himself has been saying for weeks now. "Like every American, I deserve to meet my accuser, especially when this accuser, the so-called 'Whistleblower,' represented a perfect conversation with a foreign leader in a totally inaccurate and fraudulent way," he tweeted at the end of September.

The logic behind this we-have-to-know-and-meet-the-whistleblower-or-this-whole-thing-is-a-sham argument is both deeply flawed and shows a fundamental misunderstanding of how the whistleblower system actually works.

So, let's start there. When the whistleblower filed his or her eight-page complaint alleging, among other things, that Trump had pressured Ukrainian president Volodymyr Zelensksky to look into unproven allegations against Joe Biden and his son, Hunter, on a July 25 phone call, it was the start of an investigation, not the end of one.

Whistleblowers, in corporate America and government, provide leads for investigators to look into. They are the start of an investigative process, not the end of one.

What comes next is an attempt by investigators -- Congress, the Securities and Exchange Commission etc. -- to corroborate the allegations made by the whistleblower. Are they simply a disgruntled employee trying to settle a score? Or are they someone with real and valid concerns centered around provable facts?

What's become clear over the six weeks between the whistleblower complaint becoming public in September and this week's set of public hearings is that the whistleblower knew what they were talking about.

From State Department official George Kent to National Security Council Ukraine specialist Alexander Vindman to former Ukraine Ambassador Marie Yovanovitch, there has been considerable corroboration of the whistlelower's complaint. Yes, there was concern even from those on the call -- namely Vindman -- that it was not appropriate for Trump to push Zelensky to investigate the Bidens even while military aid from the US was being held up. Yes, Trump lawyer Rudy Giuliani was running a sort of shadow foreign policy in Ukraine at the behest of Trump. Yes, Gordon Sondland, the ambassador to the European Union, told a top Ukrainian official that the security aid was linked to investigations into Biden and the 2016 election.

In the coming weeks, several of these witnesses are expected to testify to their accounts publicly. And they have already testified to these facts under oath. Lying under oath is a federal offense; lying to the media -- or on Twitter -- isn't.

Why then, given everything we have learned that affirms the contents of the whistleblower complaint, would Graham have to know who the whistleblower is in order to even consider the possibility of impeaching Trump?

Either the claims made by the whistleblower are borne out or, well, they aren't. And based on everything we now know, the whistleblower's complaint has been spot on.

Which makes the claims by Graham (and Trump) very odd. Especially when you consider that anonymity is absolutely essential to the process of whistleblowing. If you expect to be outed if you come forward with an allegation of wrongdoing, how likely are you do to do so? Answer: Not very.

The Republican arguments against impeachment have grown increasingly difficult to make -- and sell -- amid the release of closed-door transcripts that quite clearly bolster the claims made by the whistleblower complaint.

But to argue that Trump can't be impeached or that impeachment can't even be considered unless and until the whistleblower is named and interviewed, overlooks lots and lots of facts that have come to light since the original complaint.