Can you really teach an old dog new tricks? And isn't it ageist to suggest that a 76-year-old U.S. senator might be considered an "old dog"? Like almost everything that Vermont Sen. Bernie Sanders does since he exploded into America's political consciousness three long summers ago, there are always more questions than answers.

Consider the biggest question that looms over Sanders' wild grey mane — could America really elect a by-then-79-year-old democratic socialist as its 46th (or 47th or 48th … who knows these days?) president on November 3, 2020? Perhaps — especially when a veteran politician who's been around since the civil rights protests of the 1960s shows he can still learn from his recent mistakes.

Just ask John Fetterman, one of the contenders in the heated Democratic primary race for Pennsylvania lieutenant governor. In 2016, Fetterman was an upstart, left-wing primary candidate for U.S. Senate who endorsed Sanders, attended his local events, and had enthusiastic support from many Sanders voters — who were then stunned when the Vermont senator totally blew off Fetterman. (In this space, which was normally quite friendly to Bernie and his democratic socialism, I lashed out at Sanders and the case of "The Man That Bernie Sanders Forgot"). It was Exhibit A that spring for the case that Sanders was too focused on himself to spend his political capital on building a wider movement.

On Friday, Sanders returns to Pennsylvania for the first time in this critical election year of 2018, and guess what his first stop will be? A rally at Philadelphia City Hall where the progressive icon will endorse Fetterman in his lieutenant gubernatorial bid. He'll do the same later this weekend for two Democratic congressional candidates, Jess King in Lancaster and Greg Edwards in Allentown. It's part of the Our Revolution movement that's doing the hard work of planting those grassroots seeds that Sanders mostly shunned in 2016, and which already has seen a lot of success endorsing candidates like new Philadelphia DA Larry Krasner.

The rallies are an exclamation point on a huge development that's happened under the radar of the daily assaults of Trumpism. The Democratic Party is, at least spiritually, Bernie Sanders' party right now — despite the irony that the lifelong independent will never change his own voter registration to "D." True, the insiders — including current DNC chair Tom Perez — are largely still Clintonistas, but that can disguise the fact that the ideas that struck some as too radical for the timid Democratic in 2016 — universal college access, single-payer health care, a $15 minimum wage — are now party orthodoxy in 2018.

Turn on your TV and watch the ads for the slew of congressional primary candidates, and you'll see that a dramatic expansion of access to college and some version of truly universal health care — ridiculed just two years ago as too idealistic or too expensive or both — are now the price of admission for the Democratic electorate. And why wouldn't they be? Polls show that a majority of American voters (including large numbers of Republicans) now support state programs to make college tuition free (62 percent in the summer of 2016, after Sanders had hammered the issue on the campaign trail) and single-payer health care (by a slimmer 51 percent, early this year) — two key tenets of Sanders' no-longer-"radical" scheme for democratic socialism. It's no wonder so many candidates are jumping on the bandwagon.

But Bernie Sanders hasn't rested on those gains. Last month, he was back with a bold new proposal: A guaranteed job for every American, and this time other prominent Democrats were eager to stand behind him.  Liberal voters have learned — to steal Salena Zito's famous line about Trump — to take Sanders seriously but not literally, that his proposals may be utopian but in the reality-based world he'd fight for whatever fraction of them can be achieved. The jobs pitch was another reminder that — at the very moment conventional wisdom expected a politician in his mid-70s to slow down — Bernie has hit the gas.

He has never really stopped campaigning, reaching out to working class whites at town hall meetings in the heart of West Virginia Trump country but — more importantly — building new relationships with black voters in the South, where ran so poorly in 2016. His ideas and his energy, and his newfound willingness to elect like-minded candidates, make him again a serious contender for the presidency.

Some surveys of both rank-and-file voters and key party insiders have shown Bernie Sanders to be the frontrunner for the 2020 Democratic nomination, although other recent polls have shown a surge for former vice president Joe Biden (who's a sprightly 75 … call it a youth movement) or Elizabeth Warren (who turns 69 next month). It's certainly plausible to see a Sanders path to the nomination. But the chattering classes in D.C. have long maintained that Sanders can't win a general election, and that if he somehow did he'd be a lousy president, unable to make deals over on Capitol Hill.

The second part of that strikes me as wrong. I've written biographies of two politicians — Ronald Reagan and Bernie Sanders. Obviously. the two men are polar opposites politically, but there's a critical similarity between the two. Both men never compromised their core values — the Gipper's bedrock conservatism, or Sanders' focus on liberalism for the working class — yet they've governed pragmatically when the situation required it.

When necessary, Reagan cut deals with Democrats and raised taxes to deal with soaring deficits and to stabilize Social Security. Likewise, Sanders governed the city of Burlington as a practical progressive who could reach out to former adversaries and was hailed by Time as one of America's best mayors. I suspect he'd surprise a lot of people as a similarly results-oriented chief executive of the United States. In fact, I think Bernie would make a very good president, quite possibly a great one.

But I am worried about the politics involved in electing Sanders in 2020 and performing the most critical task that America has faced in my lifetime: Removing Donald Trump and his dangerous authoritarianism from the White House. It's hard to make the kind of journey that Sanders has made without picking up baggage along the way. Some of that is the Joe McCarthy-style red-baiting that would have happened if Sanders had defeated Clinton for the nomination in 2016, which I think he could brush off. Some of it seems more complicated.

Nothing has energized the Democratic base since the start of 2017 more than the Trump-Russia scandal (which, for example, has single-handedly made The Rachel Maddow Show the highest rated show on television), and so some will surely hammer Bernie over the fact that the same Russian troll farm that worked to help Trump posted pro-Sanders material online as well, even if in this case there really is zero evidence of any collusion.

Then there's the matter of Bernie's wife Jane and her problematic presidency of Burlington College, after which the institution went out of business. It appears to have been a case of poor management and not a crime, which of course didn't stop a local GOP activist (who even knew there was one in Vermont?) from referring the matter to the U.S. attorney. So it's a clearly a thing that state-run television, also known as Fox News. would make into the endless "Hillary's emails" of 2020, even though it has no bearing on how Bernie would govern (not unlike Hillary's emails)..

And if you spend too much time on Twitter — to this, I plead guilty — it's impossible to ignore that some key Democratic influencers aren't fans of Bernie and that some despise him with passion, blaming his insurgent candidacy and any discord that it sewed in the spring of 2016 for Clinton's eventual defeat. This sentiment seems particular strong with women who were the most diehard Hillary fans — like members of the group Pantsuits Nation, who morphed into the leadership of the Trump Resistance — and also some non-white voters. Personally, I think the lingering resentment is unfair and misguided, but it would be foolish to pretend it doesn't exist, and that it divides the party. And a divided Democratic Party is how we got in the mess in the first place.

When Sanders turned 75 in September 2016 and Trump was elected two months later, there was a sense that Bernie hadn't attained the presidency but had done something equally amazing in starting a bona fide political revolution — and that these four years would be about passing that torch to a new generation. The fantasy was that a candidate would emerge who embraced Sanders' ideas and his energy, but with youth and a broader appeal to those groups that didn't warm up to him in 2016. My crazy idea for saving America after Trump was"'Bernie-ism" beyond Bernie.

It may have been just that, a fantasy.