I am old enough to recall other wrenching presidential transitions:  particularly the return of Richard Nixon in 1968 and the rise of Ronald Reagan — horrors, an actor — in 1980. But both Nixon and Reagan boasted governing experience and had campaigned in traditional fashion.

Not so, of course, with President-elect Donald Trump. And as we embark on a perilous journey, testing the resiliency of American democracy, we need to retain a sense of proportion.

The first reality is that Hillary Clinton lost fair and square. It is admittedly tempting to wallow in what-might-have-beens, including a less ham-handed FBI director.

But there is scant evidence that Bernie Sanders (a candidate whose voting record would have offered the Republicans a target-rich environment) would have inspired greater African-American or Latino turnout. And dismiss the fantasy that Clinton could have found within herself a campaign theme more inspiring than “I’m-not-Trump” and breaking the glass ceiling.

There is nothing new about the current internal Democratic policy wars over whether to tilt toward Wall Street or economic populism. Bob Woodward in “The Agenda,” his account of the early days of the Clinton administration, depicts the president-elect sputtering in early 1993, “You mean to tell me that the success of the program and my re-election hinges on the Federal Reserve and a bunch of f—— bond traders?”

Ever since, for better or worse, the Democrats have tried to be the party of fiscal responsibility. What that meant for Hillary Clinton was that she felt constrained by budget realities and unable to offer a single bold program that would define her presidential campaign. Even Clinton’s tuition-free college plan — largely inspired by Sanders — came with enough caveats and complications so that it was more another laundry-list issue rather than a clarion call to anything.

[Opinion: Hillary Clinton’s Challenge – Winning the Battle of False Equivalence]

Before the Democrats embark on a purge of anyone who ever held a fundraiser at a hedge fund, it is worth remembering the limits of political prophesy. No one saw Trump coming and few envisioned the Democratic collapse in the Rust Belt. So it is impossible to know what issues will arouse voters in 2018 or 2020.

But liberals do have to ask themselves a basic question: Why is America mostly accepting gay marriage and moving toward the legalization of marijuana at the same time when there is massive resistance to new federal programs? Why, in essence, have liberals won the culture wars and lost the governing wars?

For the first time since Bill Clinton’s nomination in 1992, the Democrats are poised to pass the torch to a new generation. Joe Biden, Bernie Sanders, Nancy Pelosi and Jerry Brown — the party’s most successful governor — are all older than Hillary.

The challenge for the Democrats’ next leader (and, no, it can’t be Barack Obama) is to figure out how to make the party more than just a loose confederation of interest groups, each with its own pressing set of social issues. And rather than running from Big Government (which was Bill Clinton’s response to Reaganism), Democrats have to make it relevant to a new generation of voters.

But first, there is the reality of President Trump.

The best that anyone can realistically hope for is that Trump will govern as a mainstream conservative Republican with occasional weird Twitter wars and temper tantrums.

What that would mean, in practice, would be a fifth conservative justice on the Supreme Court, an end to Obamacare, budget-busting tax cuts, a slash-and-burn approach to government regulations and an indifference to global warming.

That’s roughly the agenda that President Marco Rubio, Jeb Bush or John Kasich would have followed. It is what happens in a democratic society when you lose an election as Hillary Clinton did.

[Opinion: The Three Voters Who Helped Me Understand Trump]

But it is the potential for a different kind of President Trump that should make everyone quake. This would be an elected leader who rejects democratic norms, surrounds himself with incompetents and cronies, uses the levers of government to punish his enemies, and makes us all nostalgic for the disastrous foreign policy leadership of George W. Bush.

If it is this Trump the Terrible who emerges from the chaos of transition, the Democrats have only one choice — reach out to Republicans of good will who are equally appalled by the careening conduct of the 45th president.

Partisanship should not blind liberals to the political courage of the “Never Trump” Republicans like Sens. Ben Sasse, Jeff Flake, Lindsey Graham, Susan Collins and belatedly, John McCain. And with only a three-vote GOP majority in the new Senate (counting Mike Pence), it should be possible to forge alliances to rein in the worst excesses of the nation’s first reality-show president.

But that requires liberals to temper their expectations and dial down their rhetoric on those days when Trump is merely wearing his conventional Republican garb. If every Trump appointee is denounced as a “racist” or a “sexist,” what part of the English language would be left if the new president tries to register Muslims? Or if the president wants to stifle a free press with draconian libel laws?

Little that has occurred so far during Trump’s purge-filled transition is reassuring. But with Sen. Rand Paul sounding off against would-be Secretary of State Rudy Giuliani, and Graham warning against deporting the young, there are already signs of an independent Republican Party.

In the long struggle ahead to preserve the constitutional handiwork of James Madison and Alexander Hamilton, liberals should welcome all allies, regardless of party.

Roll Call columnist Walter Shapiro is a veteran of Politics Daily, USA Today, Time, Newsweek and the Washington Post. His book on his con-man great-uncle was just published: “Hustling Hitler: The Jewish Vaudevillian Who Fooled the Fuhrer.” Follow him on Twitter @MrWalterShapiro.