RUTHFULLY YOURS

Begin with the objective conditions. The first is continuing public disenchantment with political, media, financial and cultural establishments. It is this disenchantment that brought Mr. Trump to the White House in the first place and, additionally, almost brought Sen. Bernie Sanders, not even a Democrat, the Democratic presidential nomination.

In Mr. Trump’s case, voters knew he was boorish, narcissistic, a business and financial freewheeler, a womanizer, and largely ignorant of governance and public policy. His election was wholly about disillusion with the alternatives. His former personal lawyer was no doubt right in asserting that Mr. Trump never expected to be taken seriously as a presidential candidate but ran simply to burnish his brand. Similar populist disenchantment, by the way, is plaguing establishment politicians in the U.K., France, Germany and elsewhere.

The other objective conditions—the two most important in a national general election—are those relating to national security and the economy. Ordinary voters see that Mr. Trump has destroyed the ISIS caliphate in the Middle East, has plans for phased withdrawals of American forces from Syria and Afghanistan, has challenged Russia and Iran, and is making an effort to denuclearize North Korea. They also see him attempting to confront China for its dishonest trading practices.

They may not support his Mexican wall as first proposed, but they recognize the need for border security. They also support American citizenship for immigrants who proceed lawfully. They puzzle that Democrats, rather than focusing on means to legalization, instead are attacking Immigration and Customs Enforcement and the Border Patrol as child torturers. They do not see or comprehend the damage Mr. Trump has done to multilateralism, alliances, carefully built international institutions, or thoughtful internal policy-making processes. They see only the externals.

The economy remains strong and unemployment at record lows, including for minorities. This growth will not last forever but may continue through the 2020 electoral cycle. The longer-term outlook is overshadowed by $22 trillion in federal debt, expanding by some $1 trillion annually, and additional debt and liabilities at state and local level. Neither political party is addressing this slow-motion crisis.

Given these objective conditions, cascading attacks on Mr. Trump and his family for prepresidential conduct are counterproductive. So are portrayals of the country as mired in economic distress and plagued by white nationalism. The country has made enormous leaps forward in racial justice over more than a half-century; white nationalists are a small and inconsequential force. If Democratic candidates persist in 2020 in labeling Republicans as antiblack, anti-Latino, antiwoman, anti-LGBTQ, anti-immigrant and anti-middle-class, they can credibly be counterattacked for diverting attention from their own lack of a credible alternative agenda addressing the core national-security and economic issues. That agenda must be serious and not based on a Green New Deal that, however well-intentioned, is currently impossible technologically and economically, is crushingly unaffordable, and contains provisions wholly unrelated to a transition from fossil fuels.

Now about the political fundamentals. When voters consider changing presidents, they usually focus on the weaknesses of the incumbent and vote for a replacement they perceive as his opposite. That has been the case in every president-changing election from 1932 until the present day. An optimistic, upbeat Franklin D. Roosevelt replacing a dour, bureaucratic Herbert Hoover; an energetic, charismatic John F. Kennedy replacing Richard Nixon, vice president in a tired outgoing administration; a thoughtful and bold-talking Nixon succeeding Lyndon B. Johnson, associated with grinding and divisive war; a morally upstanding outsider, Jimmy Carter, replacing Gerald Ford, who had pardoned the discredited Nixon; an upbeat Ronald Reagan replacing Mr. Carter, who blamed the American people for their “malaise”; and a young, vigorous Bill Clinton defeating an older, tired George H.W. Bush. Voters thus will be looking for a 2020 Democratic presidential candidate who is reflective, experienced, a unifier rather than divider, and demonstrably capable of serious governance. In other words, not just another pugnacious self-seeker.

How many of the announced Democratic aspirants fit that description? Most thus far appear to be appealing mainly to diehard anti-Trumpers important in early primary and caucus contests. But it’s a mistake to believe that general national opinion conforms to that of party activists. You could ask Presidents Goldwater and McGovern about that mistake.

House Democrats are doing Mr. Trump a favor with their Jacobin attacks on him. The Robert Mueller report is forthcoming. It will or will not provide a basis for actions against Mr. Trump via due process and the rule of law. In the meantime, Democrats should presume nothing about its content. They should instead get on with contesting the election the way we did back in the day. That means asking: What are the country’s big problems? What are our proposed solutions to those problems? How can we persuade a majority of the country and Congress to accept our solutions?

Thinking of running for president? Consider those questions now—before declaring your candidacy.

Mr. Van Dyk was active in Democratic national policy and politics for 40 years. He is author of “Heroes, Hacks and Fools” (University of Washington Press, 2007).