WASHINGTON -- "He's been lying all week, he's been lying for years. ... I don't see how he has any credibility."

Exactly. Rudy Giuliani, the president's current lawyer, was talking about Michael Cohen, the president's former lawyer. But if Cohen is so sleazy, and I don't disagree, why did Donald Trump keep the self-described fixer around for so long?

As recently as April, Trump was calling Cohen a "good man." As recently as May, Giuliani called Cohen "an honest, honorable lawyer." Cohen's character didn't change. The damage he could inflict on Trump did.

And here's the bigger problem with Giuliani's argument against Cohen: It applies to his own client. Trump lies -- constantly, flagrantly, provably. You might think that a smart lawyer, capable of seeing around a looming corner, would think twice before labeling someone else a "pathological liar."

Especially since Trump's lies include the very subject on which Giuliani now claims that Cohen's alleged account should be discounted and Trump's credited. If Cohen tells you the sky is blue, check the color. In a swearing contest between Cohen and Trump about the Trump Tower meeting, Cohen's word alone isn't reliable. Still, if there are motives to lie on both sides, whose is bigger? I'd wager the one whose presidency may hinge on the outcome.

And the one who has the longer track record of prevarication on this topic. Trump is implicated in -- he is the architect of -- the original lie, about the contents of the Trump Tower meeting with a Russian lawyer. It was adoption, sure! Trump and his lackeys then lied about the lie, denying Trump's involvement in writing the initial misleading statement about it.

"I do want to be clear that the president was not involved in the drafting of the statement," Trump lawyer Jay Sekulow said last July. That untruth was quickly overtaken by misleading spin from White House press secretary Sarah Huckabee Sanders: "The president weighed in, as any father would." Any father with zero fealty to the truth. Any father with political -- and perhaps criminal -- exposure of his own.

It is no stretch, then, to imagine Trump lying about whether he knew in advance about the meeting he then lied about. In fact, that would be perfectly in character. Sometimes it is hard to understand why Trump bothers to lie, when the truth is so provable and obvious. But often Trump's lies simply reflect the primitive instinct of the cornered toddler with chocolate smeared on his face, insisting that he had not just raided the cookie jar.

Deny first, with accompanying, if not convincing indignance. Deal with the consequences later. As Giuliani said of Cohen, "If his back is up against the wall, he'll lie like crazy. Because he's lied all his life."

We saw this on display with Trump's handling of the Cohen-facilitated payments to porn actress Stormy Daniels. Asked in April about whether he was aware of the $130,000 to ensure Daniels' silence, Trump denied any knowledge. "You'll have to ask Michael Cohen. Michael is my attorney." The next month found Giuliani and Trump himself acknowledging that Trump had reimbursed Cohen for the payment he had just denied knowing about.

Now comes Trump, on Twitter, doubling down on his denials about the Trump Tower meeting. "I did NOT know of the meeting with my son, Don jr. Sounds to me like someone is trying to make up stories in order to get himself out of an unrelated jam (Taxi cabs maybe?)."

Maybe. A smart prosecutor will seek corroborating witnesses -- Cohen reportedly says they exist -- and other indicia of truthfulness. In this case, what were the blocked numbers to which Donald Trump Jr. placed calls at key moments before and after the meeting? What motivated Trump to announce plans, the very day on which the meeting was arranged, for a "major speech," never given, about "all of the things that have taken place with the Clintons"?

What would it mean if Trump knew about and approved of the meeting with a "Russian government attorney," as Natalia Veselnitskaya was described? It would show that Trump, like his son ("If it's what you say I love it"), was eager to obtain dirt on an opponent being peddled by a foreign agent. "Most people would have taken that meeting," the president said last year. "It's very standard."

The dangled dirt was not forthcoming. That doesn't excuse a presidential candidate for seeking to procure it. Nor does it excuse a president for lying to the country for months about his role in this sordid transaction.

(c) 2018, Washington Post Writers Group