Then-candidate Donald Trump during a rally in Richmond in June 2016. (Jabin Botsford/The Washington Post)

One of the assets President Trump enjoyed during the 2016 campaign was he was broadly unknown. Not in the literal sense, of course. Any American who had watched “The Apprentice” or who had been around a New York Post from 1980 to 1995 was very aware of who Trump was. But the Trump with whom America was familiar, to the extent this image of Trump was even accurate, told us very little about Trump the potential president.

He had no background in public service that could explain what he probably would do in office, and his campaign-trail proposals were often unabashedly skeletal. Trump was a mostly blank piece of Trump Organization letterhead on which various chyrons from Fox News broadcasts had been scribbled. Republicans who were not in the bag because of those conservative-media slogans saw the white space on the rest of the sheet of paper and imagined what might be written in.

We have a much more complete picture of Trump now than we did then. We know about his business. We know about his associates. We know about his foundation, his inaugural committee, his campaign, his family. We do not know everything, by a long shot, but we are starting to get at least a better idea of what we do not know.

As we have learned more about Trump, one article of faith has been reinforced over and over again: Republicans are sticking with him.

Consider a poll question from Monmouth University, published this week.

“In general,” the pollsters asked, “do you think there is any new information that could ever come out about President Trump that would significantly change public opinion of him, or do you think people are set in their opinions regardless of what new information may come out?” This question followed a battery of questions about former Trump attorney Michael Cohen’s testimony before a House committee last week, but it is itself broad. It is not about Russia or Trump’s business: It is about anything.

Three in 10 respondents said some information could come out that might change public opinion on Trump. Two-thirds said nothing could move opinion numbers. Among Republicans, though, 1 in 5 thought something might emerge to change public opinion on Trump, and more than three-quarters said it would not.

(Philip Bump/The Washington Post)

That is significant: The people who are most loyal to Trump are least likely to think anything will change people’s minds about him.

Mind you, this is somewhat at odds with a PRRI poll released in December 2017. At the time, about half of Americans reported that they themselves were unlikely to change their minds about Trump, including about a third of those who approved of him. This was people talking about themselves, not, as in the Monmouth poll, considering the country as a whole.

(Philip Bump/The Washington Post)

But poll data suggest it is probably safer to assume solidity among Republicans than that two-thirds of Trump’s support is more malleable. (Some of Trump’s support comes from independents, of course. Here we are looking only at Republicans.)

The polling firm YouGov provided The Washington Post with its daily job approval numbers for Trump since February 2017. The distribution of approval among Republicans is very narrow: More than half of the days for which YouGov has polling, Trump’s approval rating among Republicans has been between 80 percent and 85 percent.

(Philip Bump/The Washington Post)

Notice, by the way, the similarity of the three-quarters of Republicans who think views of Trump will not change with the constant 80-odd percent of Republicans who view him with approval. There is substantial overlap between those groups.

That solidity of support looks like this when viewed as a more traditional line graph. Below we have taken seven-day rolling averages to flatten out some of the ups and downs.

(Philip Bump/The Washington Post)

There are two periods of slow change. The first comes at the outset of Trump’s presidency, with a slow slide that speeds up in mid-April to late May of 2017. Then there is a gradual upward climb from November 2017 to November 2018. But these shifts are pretty subtle.

There is another metric provided by YouGov that gets closer to the question of how variable Republican support might be. If we consider what percentage of Republicans indicate they approve of Trump strongly, the pattern is a bit more spread out.

(Philip Bump/The Washington Post)

In fact, considering this metric allows us to see up-and-down movement that occurs at certain points in time.

(Philip Bump/The Washington Post)

The first is that drop at the outset of his presidency in 2017, probably a function of the traditional honeymoon period presidents get having worn off. Then that slow climb starts, spiking from mid-April to late May. Why? It is not clear. It could have been a reversion to the trend after early April — a period in which Cohen’s home and office were raided by federal authorities.

The big spike came last fall, in the run-up to the midterm elections. Trump was campaigning constantly, often mentioning the confirmation of Supreme Court Justice Brett M. Kavanaugh following a heated fight in the Senate at the beginning of October. After the campaign, the shutdown fight began.

But the number of Republicans who said they strongly approved of Trump started tanking before the shutdown — right after the midterms, in fact. This, some have posited, was a function of disappointment over Republicans losing the House.

In February, Trump’s strong approval numbers climbed again, after the shutdown ended, but then sank back down over the past few weeks.

Notice two things about all of this.

First, Trump’s strong approval numbers do not appear to have been particularly affected by things like the racist incident in Charlottesville in August 2017 or the ongoing developments in special counsel Robert S. Mueller III’s investigation of possible Russian links to the Trump campaign.

Second, even when his strong approval has softened, Trump’s overall approval really has not. In other words, the shift in support for Trump among Republicans has largely been one of degree.

This issue has little to do with the 2020 presidential race. Trump is now and always has been likely to win most Republican votes in that election as he did three years ago. That blank letterhead has been filled in with things that are specifically designed to keep Republicans happy.

It simply means most Americans and most Republicans are probably right: Whatever else is to come from Trump’s presidency, his base will stay his base.