A key trope of any inaugural address is a pledge of unity — that, despite the divisions of the campaign just past, the new president will serve the entire nation and not the interests of one political party. Thomas Jefferson probably put it most memorably in 1801, during the first inauguration that marked a transfer of partisan power:

Let us, then, fellow-citizens, unite with one heart and one mind. … [E]very difference of opinion is not a difference of principle. We have called by different names brethren of the same principle. We are all Republicans, we are all Federalists.

Just four years ago, in 2013, President Obama reminded “my fellow Americans” that the oath of office is “an oath to God and country, not party or faction. And we must faithfully execute that pledge during the duration of our service.”

Donald Trump will probably include something similar in Friday’s addition to this canon, as he did when he claimed victory in the predawn hours of Nov. 9. Then he struck a conciliatory tone, saying:

To all Republicans and Democrats and independents across this nation, I say it is time for us to come together as one united people. … I pledge to every citizen of our land that I will be president for all Americans, and this is so important to me.

Of course, many doubt just how important, given the past 10 weeks of divisive rallies, appointments, news conferences and Twitter fights. The new president enters office with historically low approval ratings, which have receded even from the 46 percent of the national vote he won in November.

But as I note in a forthcoming chapter in a volume of essays on the 2016 election, Trump’s situation in this regard is perhaps different only in degree from past presidents, not in kind.

Presidents claim to speak for the nation. But in practice they are more often minority leaders.

For one thing, it is relatively rare for presidents to win wide majority support at the ballot box. The divergence in 2016 between the electoral and popular votes naturally re-energized the debate about the virtues of the undemocratic (but quite Republican) electoral college.

But even in “normal” elections, presidents usually enter office with a small majority or plurality of the vote. In 1980, an election widely remembered as a landslide, Ronald Reagan earned less than 51 percent of the national popular vote. In 1896, a year sometimes treated in the political science literature as a sweeping partisan realignment, William McKinley won a whopping 51.1 percent. And so on.

In only 30 of the 49 elections from 1824 to 2016 did the winning candidate get even a majority of the popular vote. Less than a quarter of the time (12 of the 49) did the winning candidate receive 55 percent or more.

Thus, most presidents are opposed by at least a substantial minority of the voting public even at the outset of their terms — and presumably some proportion of the large nonvoting public as well. The electoral college helps to paper over these divisions, of course. Memories of Reagan’s 1980 landslide are predicated on his stunning tally of 489 electoral votes — 91 percent of the total — rather than his 50.8 percent of the popular vote. Bill Clinton received 43 percent of the popular vote in 1992, yet nearly 70 percent of the electoral college vote.

But public opinion data tend to reflect or even lag the vote returns, sooner rather than later. Trump is rare in receiving no “honeymoon” at all in this regard. Even so Obama, despite approval ratings near 70 percent upon his first inauguration in January 2009, will leave office with average public approval for his entire tenure at just 48 percent. George W. Bush’s immense jump in job approval on Sept. 12, 2001 — from 51 to 90 percent — reflected both events and his performance, but in retrospect was clearly anomalous. It dropped down steadily over time, and his second term approval averaged just 36.5 percent.

Indeed, every president from Lyndon B. Johnson through Obama saw his approval drop below 40 percent at some point — four of them (Nixon, Carter and both Bushes) below 30 percent.

Average approval over the past 50-plus years (that is, since Johnson’s 1965 inauguration) is barely 51 percent. Presidents claim to be “uniters, not dividers,” in George W. Bush’s famous formulation, but in practice they seem to be just the opposite.

They talk like uniters but act like dividers. 

More important, they act like dividers, not uniters. Any sequence of close national elections, and the related rise of polarization between the parties, further heightens the importance of the presidential “base.” One result is that presidents are increasingly less likely to reach out beyond their electoral coalition.

Consider the growing scholarly literature that suggests presidents weight both their rhetoric and their policy proposals heavily toward their partisan base. B. Dan Wood’s book “The Myth of Presidential Representation” gives away that conclusion in its very title. Wood found that presidential policy attitudes converge very little with the “national mood” in the same policy arenas for the same time frame — but very strongly with that of their partisan constituency.

Rather than change their positions to accommodate the political center, presidents try to persuade people near the center to adopt their own presidential preferences. (In this, they tend to be unsuccessful; as George C. Edwards III has found, Oval Office rhetoric often falls “on deaf ears.”)

Other studies using different data find roughly similar sorts of presidential targeting. Matthew Eshbaugh-Soha and Brandon Rottinghaus conclude that presidents’ policy proposals are more heavily weighted toward their partisan base than toward the center of public opinion by about a five-to-four margin, and that this ratio increases as polarization does. Studies by John Hudak and Douglas Kriner and Andrew Reeves show that “presidential pork” is widespread, that a “particularistic president” targets discretionary federal funds and grants toward areas providing political support.

Thus, while Trump has seemingly shown little interest in uniting the country behind his administration, any new president would have to make a significant effort to claim the status of majority leader. (This would have been true of a President Hillary Clinton as well.) Precedent suggests instead that while claiming to be the people’s tribune, the new president will define “the people” in a partisan manner. Put another way, common ground is visible only if presidents look for it — and despite their rhetoric, they rarely have reason to do so.

Such a strategy superficially reflects the anti-majoritarian roots of the presidential office — which was designed not as a bastion of majority rule, but as a check on it. But it fits less comfortably into an era in which broad public legitimacy is a crucial part of presidential power, a development so key as to be considered almost a “second Constitution.” It is yet another reason to recall a piece of wisdom from the new president’s favorite musical: “Winning was easy, young man — governing’s harder.”