The most conservative candidate running for president in 2020 isn't Donald Trump or any of his long-shot Republican challengers for the GOP nomination. It's Joe Biden, the ostensible Democratic front-runner.

This is hard for us to see because over the past half century or so in the United States — since the presidential campaign of Barry Goldwater in 1964 — the term "conservative" has been given a precise ideological spin. To be conservative is to favor economic libertarianism (low taxes, minimal regulations on business), moral traditionalism (opposition to feminism, abortion rights, gay rights), and foreign policy hawkishness (originally a willingness to use military force to contain the Soviet Union, now an eagerness to use military force to accomplish just about every goal in international affairs).

Biden obviously isn't a conservative in this sense — let alone in the Trumpian variation on the ideology, which adds hostility to immigration and a suspicion of free trade to the mix.

But the conservative sensibility is much older than the post-Goldwater Republican Party. In fact, one way to understand the American Constitution is as a grand effort to instantiate in the country's fundamental law a temperamentally conservative approach to small-r republican self-government. Yes, we have elections to gauge public opinion and act as one of several checks on overweening government power. But public opinion itself is checked and channeled into institutions explicitly designed to slow down decision making, cool down partisan passions, and encourage restraint. That's why impeaching and removing a president, like amending the Constitution itself, is hard, requiring that those behind the efforts muster supermajorities to enact their plans.

It's in this sense that Joe Biden is a conservative. Throughout his career he has taken his strongest public stands in favor of norms, rules, traditions, and institutions designed to restrain and balance competing forces in our politics. He tends not to be motivated by moral or ideological imperatives. This is abundantly clear from his long-standing record of hesitation to support impeachment as a remedy for presidential malfeasance, recently and usefully summarized in The New York Times.

Biden was first elected to the Senate in 1972, and the ever-expanding Watergate scandal dominated his first two years in office. In a floor speech delivered in April 1974, five months before Nixon resigned in the face of near-certain impeachment, Biden refused to call for this most drastic of measures against the president. Instead, he insisted on giving Nixon "the utmost presumption of innocence." The 31-year-old freshman Democrat went on:

In a day when equality is on everyone's lips, I suggest that in practice, in extraordinary times involving our highest office of government, some people may be, in fact, more equal than others. And in this case, I defer to the president. His official derelictions, if proved, and his subsequent ouster, should it occur, will have an impact on our institutions and upon ourselves as Americans, beyond that which it would have for any of us who are involved in such a proceeding. [Biden quoted in The New York Times]

Biden took this position because he sensed the gravity involved in Congress overturning the results of a recent landslide election. (Nixon had won reelection in 1972 by one of the widest margins in modern American history — with 520 electoral votes and 60.7 percent of the popular vote.) He made the same point to his Republican colleagues 24 years later, when he defended a similar position of restraint in the name of public sentiment during the drama surrounding the impeachment of Democrat Bill Clinton: "This. Is. Their. President. Don't screw with him unless you have an overwhelming case that you can make that you're being fair about it." In Biden's view, Republicans in the House failed this standard, refusing to slow down their drive for impeachment, even after Democrats picked up seats in the November 1998 midterm election. Their personal and ideological animus for Clinton was so strong that they pushed ahead anyway, overriding concern for prudence and propriety.

It's the same cluster of considerations — above all, the importance of exercising caution, appearing public spirited, and demonstrating fairness when attempting to build public support for impeachment — that has led Biden to hesitate in calling for Trump's removal, even in the face of the president's unprecedented attacks, many of them of questionable legality, on his own son Hunter Biden.

On Wednesday, Biden finally relented and joined most of his fellow Democrats in throwing his support behind impeachment (though not necessarily conviction and removal, about which he remained circumspect). Note that Biden's rationale was firmly fixed on the president's offenses against the country and its fundamental law. Trump had "betrayed this nation" by "shooting holes in the Constitution." That's why the president should be impeached, Biden declared: "to preserve our Constitution, our democracy, our basic integrity." Yet later in the day, he admitted how unsettled he remains about initiating a process of impeachment, calling it "a God-awful thing for a nation to go through."

All of this makes Biden a conservative — not in an ideological sense, but in a temperamental one. His instincts are to resist intense partisanship, to appeal to the American people as a whole, to place his trust in institutions, to seek common ground and strike deals with his opponents, and to defer to public opinion while also balancing it with other, higher considerations.

Biden may not be the only Democrat running for president who could be described as a temperamental conservative. The term may also fit Sens. Amy Klobuchar of Minnesota and Michael Bennet of Colorado, for example, but both of them have struggled to rise above 2 percent in the polls over the past six months of early campaigning. Biden himself might be languishing in the polling basement had he not served as Barack Obama's vice president for eight years, building a large reservoir of public support and affection among key Democratic constituencies.

That's because this is a populist moment in American politics (and in politics throughout much of the democratic world) — and populism is profoundly unconservative. Instead of opposing demagoguery, populism weaponizes it. Instead of seeking to cool passions, populism seeks to intensify them for political gain. Instead of deferring to established norms and institutions, populism targets them for abuse because they stymie the popular will. Instead of working to defuse polarization, populism seeks to magnify it. Instead of working to build a broad-based, cross-partisan consensus in favor of the common good, populism treats electoral opponents as enemies of "the people."

There are many reasons why Biden is struggling to maintain his early lead in the race, including his advanced age and difficulty holding his own in debates with his fellow Democrats. But the biggest reason of all may be that he is a temperamental conservative at a time when that kind of conservatism has fallen out of favor almost everywhere.