Washington (CNN) Back when I was at The Washington Post, I would occasionally engage in long email conversations about politics with my longtime friend -- and the Post's senior Capitol Hill correspondent -- Paul Kane . PK's insights were so good that we often published our exchanges so I could share his thoughts with readers. Now that I'm at CNN, I still value PK's wisdom about Congress -- and politics more generally.

So when Joe Biden embarked on his media tour for "Promise Me, Dad" -- a memoir of the year in which his eldest son died from brain cancer -- I reached out to PK to talk about the former vice president and his future political ambitions, including a potential 2020 presidential bid. Our email exchange is below -- edited only slightly for flow.

Chris: OK, PK. We may be at different media organizations but through the power of electronic mail, we can still communicate!

Let's talk Joe Biden. He's been all over the media in the last 72 hours -- he was on the "Today" show so much on Monday I thought he had been hired as a new anchor -- promoting his new book about the death of his son, Beau, and his life up until now.

In these interviews, he has the same pat response about 2020: If I had to decide today, I wouldn't run. But I am not shutting the door on it for 2020.

Two questions:

1. Do you believe him that he wouldn't run if he had to announce today? (I sort of don't.)

2. What makes up his mind -- to run or not run -- between now and, let's say January 2019? Is it external stuff? Personal things? Both? Neither?

PK: Greetings old friend. How is it working with : Greetings old friend. How is it working with Mark Preston again? OK, don't answer that question, it would involve too many expletives and would divert us from the topic at hand, Biden.

First off, it should be noted that the frame of this book -- "Promise Me, Dad" -- was crafted long before Donald J. Trump was sworn into office. It actually was crafted in expectation that Hillary Clinton would be president and Biden would be almost solely focused on the cancer moonshot.

I wrote a piece over several months in 2016 about Biden's heavy focus on stumping for Senate Democrats , trying to flip the majority last fall. On one trip, flying from Los Angeles back east in September 2016, he told me about the book proposal: a year in his life, from learning Beau's cancer was back, to watching him fight for his life, to his death, to America's outpouring and his consideration of a late-breaking bid for president, to him standing in the Rose Garden with Obama and Dr. B, saying he wouldn't run.

He wasn't even sure he could write the book, given the emotional ground he would have to plow.

So, this book itself is not the usual presidential campaign book. It was originally meant to be a bookend to his career. But life and circumstances change, sometime in a moment's notice, sometimes over several years, and no one knows that better than Joe Biden. So now he sits among the top tier of the type of people who would be mentioned in 2020. I think the answer to your two questions are unknowable, because the first scenario is actually more like a UK-style snap election -- and that's the sorta campaign Biden would be most likely to run, a brief sprint of a few months from nomination to general election. But those don't exist in America.

So, question two is more important, and I think Biden is being pretty open and pretty honest when he says that he's waiting to see how the field develops. He wants to be president, of course, he's ridiculously open about that ambition. But he also knows that change elections don't usually go to 77-year-olds who spent 44 years in federal office.

Over the next 18 months he's waiting to see if someone, even a few people, emerge as real credible contenders to Trump. Because, at the moment, no such person really exists.

Chris: Preston's new glasses deserve their own Twitter feed. (And yet, there are NO pics of him in the glasses on Google Images. I smell a conspiracy.)

On Biden's age: He would be, of course, the oldest person ever to be elected president. He would take that record from one Donald John Trump, who was elected at age 70.

Which sort of gets me to my point: We are living way longer. Our politicians, not surprisingly, are also living longer -- and running for (and staying in) office longer.

The top tier of the 2020 field is not exactly spring chickens with or without Biden. Bernie Sanders will be 79 on election day! Elizabeth Warren will be 71. Hell, Trump will be 74 on Election Day 2020!

That's my pro-Biden point.

My anti-Biden point is this: Everyone -- myself included! -- put him in the top tier of potential 2020 candidates. But, that -- admittedly early -- handicapping is based solely on the fact that he spent eight years as Barack Obama vice president. After all, Biden's own record as a presidential candidate is way less impressive. He dropped from the 1988 presidential primary in 1987 after a plagiarism scandal. In 2008, he was never close to being a serious contender and never, really, had a moment in which he appeared to be building momentum in the race.

So, aside from having been picked by Obama to be vice president, what's the convincing evidence that Biden would be a great presidential candidate befitting his top tier status?

PK: OK, I'll start with your last point/question, then work back to the age issue.

Biden belongs in the top tier both because he served as Obama's VP, giving him universal name ID, and because of his deep knowledge of foreign policy that began through a couple decades of service on the Senate foreign relations committee. Those are top-tier credentials. Furthermore, and perhaps most importantly, Biden has, better than any Democrat at the national level, cultivated an everyman image that has an appeal to voters in white working class exurban areas near Scranton and to inner-city black neighborhoods of North Philadelphia. No one will ever accuse Biden of being an elitist.

That trip I mentioned earlier, from LA to back east, that was the night that Clinton made her "deplorables" remark at a fundraiser. He had just spent 20 minutes lamenting how his party had become perceived as the party of "pedigree," the party of Ivy elites. His head exploded when we relayed to him the full quote of what Clinton had just said.

All of that gives him an appeal to the voters that Democrats lost from 2008 and 2012 to 2016 -- particularly those across the Midwest, in places like Erie, Pennsylvania; Toledo, Ohio; Macomb County, Michigan; Racine, Wisconsin.

But, but, but, you can't talk about Biden being in the top tier for 2020 without first addressing the issue of age, and as it relates to how voters will view it and also whether or not it's the right idea for the nation. Biden's own health has been pretty good for a 74-year-old, for sure, and his mother lived till she was 92.

The issue won't necessarily be whether he's fit to serve come January 2021, but what toll the presidency takes on even the healthiest of people. He turns 75 next week. In the fall of 2024, he would be presumably running for re-election at the age of 81.

Put another way, if Biden served two terms, he would turn 86 on November 20, 2028, as he was passing the Oval Office to his successor. The distance from 74 to 86 is big for any human, let alone one that would spend eight years of that span in the toughest job in the world.

I've watched, up close, as number of senators who were incredibly sharp and energetic in their mid-70s, turn very slow and plodding by their early 80s. We have to think, as a society, whether we want the leader of the free world to be challenging Father Time.

Chris: So, that's interesting. I hadn't thought of the age issue as much as you have. It's true that Biden would be super old -- that is a technical term -- if/when he ran for re-election. At the same time, Trump will be 74!

Anywho. Let's assume -- and you are convincing on this front -- that Biden is a top tier candidate along with either or both Warren and Bernie. What's your sense for what that race looks like?

My strong sense is that either or both Warren and Sanders seek to cast Biden as establishment. As too centrist. Too compromise-oriented. Clinton 2.0. (Or 3.0) (Or 4.0). (Yes, I know it is ironic that Biden would be cast as the heir to Clintonism given his stated disdain for the campaign Clinton ran in 2016.)

Do you agree? And what's your sense of how Biden would respond to that characterization/attack?

PK: To be sure, I'm not saying that Biden, or anyone in their 80s, cannot do the job. It's just a serious issue, and yes, I've seen it firsthand, in my own family, watching someone go from completely with it and snappy in their late 70s, to wheelchair-bound by mid-80s. And I've seen the drift of a bunch of senators in the last six years or so. It's tough.

In terms of how the race shapes up, Biden will get pegged as the establishment guy, no other way about it. And he'll have to embrace it, not run from it. No use pretending to be something else. Sanders and Warren will position themselves ideologically to his left on banking/financial issues, both because of his Senate background in a state that was home to many credit card companies and his general view of not trying to scapegoat people, which can sometimes sound like defending the banks. And he was an avowed defender of TPP, which Sanders vehemently opposed. (Biden's Senate record on trade is mixed, opposed to NAFTA and "yes" on normalizing trade with China.)

A key issue for Democrats, more broadly, is how they define ideology going forward. Over the last eight years, Republican voters have increasingly abandoned traditional conservative issues and policy positions and instead adopted emotion and anger as their primal ideological demand. If Democratic voters want emotion and anger and ferocity and strong denunciations of Trump more than liberal policy positions, that's a net positive for Biden. No one is ever going to accuse him of emotional moderation. No. One.

But if Democratic primary voters are looking for someone who screams "Glass Steagall" and "single payer," that's bad for Biden and would leave him in a position resembling the original Joe-mentum: Lieberman, who began the 2004 campaign atop the polls just because of his name ID from having been the VP nominee in 2000.

Chris: I don't think Biden is a re-run of Lieberman. Lieberman was VERY much a conservative Democrat. Take Biden's record as a whole and it's hard to imagine him being pegged that way. Yes, he is a compromiser -- : I don't think Biden is a re-run of Lieberman. Lieberman was VERY much a conservative Democrat. Take Biden's record as a whole and it's hard to imagine him being pegged that way. Yes, he is a compromiser -- the fiscal cliff deal he cut with Mitch McConnell in late 2012 , for example -- but on virtually every issue liberals care about, he's going to be with them.

All of this brings us to the only question that really matters: Does Biden run or not?

I think he does -- barring some sort of unforeseen health hiccup between now and the end of 2018/start of 2019. He had the presidential bug before he was the VP for eight years. And, as Mo Udall famously said, "the only cure for presidential ambition is embalming fluid."

Biden wants to be president. He thinks he'd be good at it. And he believes in his heart of hearts the country needs him.

I think he's going to be in. You?

PK: Yeah, first things first, Biden would obviously start as a much more credible candidate than Lieberman, but the 2003 Lieberman was not as out of step ideologically at that time as he was by 2010-12. My point is, overall, I think Biden's emotional nature -- just re-watch Yeah, first things first, Biden would obviously start as a much more credible candidate than Lieberman, but the 2003 Lieberman was not as out of step ideologically at that time as he was by 2010-12. My point is, overall, I think Biden's emotional nature -- just re-watch his 2016 address to the DNC in Philly , it's as good as Joe Biden has ever been in that format -- will compensate for some ideological baggage the liberals will throw at him.

Ultimately, no one really knows if he'll run, not even Dr. B, as Jill Biden is known to his staff. Go find that 1974 Washingtonian profile , his first national profile after his wife and daughter died in the Decmeber 1972 car crash. He openly admitted he wanted to be president, in his second year in the Senate.

The question -- as has often been the Biden conundrum -- is whether Biden makes a decision, a pure, definite decision. Or, as has happened more often than not in his career, Biden makes a decision ... by not making a decision.

If he's running, it will be pretty clear by late 2018. You'll see names like Greg Schultz and Steve Schale interviewing folks for jobs. Greg ran Ohio for Obama, became essentially Biden's political director in 2013 and would've likely been campaign manager had Biden jumped into the 2016 race. Steve ran Florida for Obama and never caught the DC bug, stayed down there but became part of Biden Family in 2015. He would've had a big role in 2016 and will have one in 2020 ... if Biden gets in.

But if this whole decision lingers and lingers, deep into 2019, then that means we're going through the same thing again, as happened in 1984 and 2004 and 2016. In each case, he kept thinking about running, kept talking about running, without ever actually running.

This is Biden's decision-making style on a lotta things, driving some folks mad. The act of trying to decide becomes the decision itself -- because by the time the decision is really ready to be made, it's too late to make any decision but no-go.

One thing we know, for sure, is he'll enjoy this ride. In July 2016, he bounced on stage to the Rocky theme song ("Gonna Fly Now") and delivered as good a speech as Clinton's team could've hoped. He and Dr. B headed off the stage and the crowd in Philly was in full rage. Just at the last step, he stopped, as Jill left alone, he turned around and -- thinking it was the last time he'd ever really have on a stage like that -- soaked it in for one last second, gave a final wave and left.