President Trump pardoned former Maricopa County, Ariz., sheriff Joe Arpaio Aug. 25. Here’s what you need to know. (Patrick Martin,Victoria Walker/The Washington Post)

President Trump has set his presidency on an unambiguous course for which there could be no reversal. He has chosen to be a divider, not a uniter, no matter how many words to the contrary he reads off a teleprompter or from a prepared script. That’s one obvious message from Friday’s decision to issue a pardon for controversial former Arizona sheriff Joe Arpaio.

Trump has been a divisive force from the very start of his campaign for president, a proud disrupter of the political status quo. His swashbuckling contempt for political correctness and the rules of the game endeared him to millions of Americans who were fed up with Washington, with career politicians, with liberal elites and with the mainstream media. The more he is under fire — as he is now — the more he returns to that strategy.

There is little doubt that his decision to seize on the issue of immigration, particularly illegal immigration, helped fuel his successful run to the White House. It’s an issue that resonates far beyond the nation’s southern border.

Accidentally or intentionally, Trump tapped into fear and anger over immigration that existed in many parts of the country, including Midwestern states where electoral votes gave him the presidency.

His willingness to use issues of culture and identity to rally supporters, even as his words and actions repelled critics, was a strategic success. There are other reasons he won the election, but immigration is a central factor.

Joe Arpaio's illegal-immigration crackdown made him a polarizing figure and an early ally of President Trump. (Meg Kelly/The Washington Post)

Illegal immigration long has been a divisive issue in American politics. Securing the border, fulfilling the needs of many businesses for migrant workers and deciding how to justly treat the millions of undocumented immigrants already in the country — some have been here for decades and are rooted in their communities — has defied political solution. Politicians in both parties have tried and failed for years to find middle ground and thereby tamp down the conflicts that have arisen.

Trump’s views have not been in doubt. As a candidate, he condemned Mexican immigrants as rapists and criminals. He called for a ban on Muslim immigration. He attacked a federal judge overseeing a suit against Trump University, claiming the judge could not be impartial because he was of Mexican heritage.

His policies in office have brought a sharp change from those of the Obama administration. From his entry ban aimed at several Muslim-majority countries to raids that have shaken immigrant communities, Trump has had a demonstrable impact.

It is a promise he made that he is keeping, even though he has yet to get Congress to fund the border wall that was a rallying cry during his campaign. The Trump administration is now weighing whether to end the Obama administration’s policy designed to protect children from deportation — known informally as “Dreamers” — who were brought to the country illegally, the program known as Deferred Action for Childhood Arrivals (DACA).

In pardoning Arpaio, the president has again linked himself to the most extreme elements of the immigration debate, inflaming an already highly volatile situation. The pardon was an extraordinary act coming so early in a presidency and sets a tone both on immigration and on the president’s willingness to use this power to take care of those who have been loyal to him. That is something that could come into play in the future, depending on the outcome of the investigation into Russian meddling in the 2016 election.

Arpaio made his reputation as an uncompromising sheriff whose harsh treatment of undocumented immigrants over many years drew criticism, condemnation and eventually legal action. He became a national symbol in the immigration debate — loved, reviled and unrepentant. He defied a judge’s order to stop racially profiling Latinos in his state. When he kept on with the practice, he was eventually convicted of criminal contempt, a misdemeanor. Now he has been pardoned.

President Donald Trump gestures to the crowd while speaking at a rally Tuesday in Phoenix. (Rick Scuteri/AP)

Once the speculation about a pardon for Arpaio surfaced, the endgame was never in doubt. It was certainly not in question after what Trump said during his angry, off-script performance in Phoenix on Tuesday night. He wouldn’t say the exact words at the time and would not use the rally to announce the decision, but his intention was clear. “I’ll make a prediction,” he said of Arpaio that night. “He’s going to be just fine.”

Nonetheless, when the pardon came down, the decision created a fresh controversy for a president already embattled. Sen. John McCain (R-Ariz.), who has felt the political blowback of anti-illegal-immigration forces whenever he has tried to help craft a comprehensive legislative solution, and who has tangled regularly with Trump, was one of the first to condemn the move.

“Mr. Arpaio was found guilty of criminal contempt for continuing to illegally profile Latinos living in Arizona based on their perceived immigration status in violation of a judge’s orders,” McCain, who is being treated for brain cancer, said in a statement released Friday night. “The president has the authority to make this pardon, but doing so at this time undermines his claim for the respect of rule of law as Mr. Arpaio has shown no remorse for his actions.”

What was perhaps unexpected was the timing of the pardon. For starters, it came only days after the president had delivered a speech about national unity before the American Legion in Reno, Nev.

That speech was Trump’s latest effort to undo the damage from his multiple statements after the white-supremacist march in Charlottesville on Aug. 12. Criticized for failing to unequivocally condemn neo-Nazis and the Ku Klux Klan at the time, he has tried both to rewrite the history of what he actually said and issue calls for unity.

In Reno, he called on everyone to help “heal the wounds that have divided us, and to seek a new unity based on the common values that unite us.” The pardon for Arpaio put that speech into fresh context. Actions speak louder than words.

What also was unexpected was that Trump decided to announce the pardon as Hurricane Harvey was bearing down on Texas. At a time when the public would expect the president to stay fully focused on the well-being of people in harm’s way of a powerful storm, he chose to divert the country’s attention by stirring controversy elsewhere.

Ever since his election, Trump has had the opportunity to try to expand his coalition, to reach beyond his base and to increase the size of his governing constituency. His electoral margin was comfortable enough, but three of the states that tipped the balance — Pennsylvania, Michigan and Wisconsin — were decided by less than a percentage point, and he lost the national popular vote to Hillary Clinton. That’s a fragile basis from which to govern.

Given those realities, a prudent politician presumably would seek ways to draw more voters into his or her orbit. Trump consistently has done the opposite, with actions designed to bind himself ever more tightly to the constituency that elected him at the cost of permanently losing potential supporters. The Arpaio pardon fits that pattern in bold colors.