Donald Trump spent a lot of time raising doubts over President Obama's birth certificate in 2011 – but now he won't clarify what he really believes. (Peter Stevenson/The Washington Post)

Donald Trump was the mainstream face of a fringe movement five years ago that questioned Barack Obama’s place of birth and qualifications for office, eventually prompting the president to release a long-form certificate proving his birth in Hawaii.

Trump has never apologized, recanted his charges or even admitted error. Instead, he tries not to discuss it.

“I don’t talk about it because if I talk about that, your whole thing will be about that,” Trump told reporters aboard his plane on Labor Day. “So I don’t talk about it.”

This is a persistent pattern for the stubbornly unapologetic Republican nominee.

Judging by his own words, the Republican nominee apparently still believes that prisoners of war such as Sen. John McCain (R-Ariz.) are not heroes; that a federal judge of Mexican heritage is unable to fairly rule in a civil case involving Trump University; that the Muslim American father of a fallen soldier has “no right” to question him; that former president George W. Bush should be blamed for the 9/11 terrorist attacks; that foreign Muslims must be barred from entering the country until officials “figure out what is going on”; and that the father of Sen. Ted Cruz (R-Tex.) might have been involved in the murder of President John F. Kennedy.

The approach has often allowed Trump to dodge responsibility for attacks, part of an attempt to portray himself as a more moderate candidate in the general election.

But it has also provided a stream of material for Hillary Clinton and her Democratic allies as they compose attack ads. One released this summer showed small children observing some of Trump’s most offensive comments, including when he mocked a disabled reporter, while another released this week featured veterans listening to Trump insult McCain and claim to have sacrificed for his country.

The Trump campaign declined to answer a written series of questions about where the candidate now stands on a long list of positions or insults. Instead, campaign spokesman Jason Miller pointed to a blanket expression of regret that Trump made last month and then provided a list of controversies that he believes Clinton should explain.

“When Hillary Clinton chooses incorrectly, there are real world ramifications, presenting a troubling pattern that questions whether or not she’s even qualified to be President of the United States,” Miller wrote in an email.

The Clinton campaign has been trying to hold Trump to his most definitive positions — a task that can be difficult. Clinton deputy communications director Christina Reynolds said that on a variety of issues, Trump’s imprecision or his failure to address certain topics are attempts to “have it both ways.”

“He was birther for years and years and years, and now he doesn’t talk about it,” Reynolds said Tuesday. “If he doesn’t believe it anymore, he should say so.”

Even when Trump seems to regret some of the things that he has said, done or tweeted — like when he retweeted an unflattering photo of Cruz’s wife, questioned former rival Ben Carson’s religion or used the word “bimbo” in a tweet about Fox News’s Megyn Kelly — he will stop short of the genuine apology that is common in American politics. Last summer, Trump said he couldn’t remember if he had ever asked God for forgiveness and in September said that “apologizing’s a great thing, but you have to be wrong.”

Trump seemed to come close to acknowledging the potential for error when he said at a rally in Charlotte last month that sometimes during the heat of debate he will choose the wrong words or say the wrong thing.

“And believe it or not, I regret it,” Trump said. “And I do regret it, particularly where it may have caused personal pain.”

But when pressed since then to explain what he regrets, Trump and his top aides have declined to provide examples.

“He has said that he wants to regret anytime he’s caused somebody personal pain by saying something that he didn’t intend to cause personal pain, and I think those who have received it privately should take that expression of regret,” Trump’s campaign manager Kellyanne Conway said on ABC News last month. At another point, she said, “He’s expressed his regret publicly and said if I have caused you personal pain — that can include me, that can include you — that he regrets that.”

During an interview with Fox News’s Sean Hannity that aired on Aug. 24, Trump reiterated that he does “regret things” but did not provide any examples and said that he doesn’t regret calling Cruz “Lyin’ Ted” or Clinton “Crooked Hillary.” Then he changed the topic to the “tremendous success” of his “tremendous campaign.”

Trump began to build his current political brand in 2011 by questioning Obama’s qualifications for office. Trump never came out and said where he thinks the president was born, but he demanded to see the president’s full birth certificate and other documents, forcing the fringe issue into the mainstream.

In April of that year, Obama released his long-form Hawaiian birth certificate in the name of putting all of the conspiracy theories to rest, and Trump congratulated himself.

“Today, I am very proud of myself because I have accomplished something that nobody else has been able to accomplish,” Trump said at the time. “I was just informed, while on the helicopter, that our president has finally released a birth certificate. . . . He should have done it a long time ago.”

But Trump didn’t let the issue go. In June 2015, CNN’s Anderson Cooper asked Trump whether he now believes Obama was born in the United States, and Trump didn’t clearly answer.

“No. I don’t know. I really don’t know,” Trump said. “I mean, I don’t know why he wouldn’t release his records. But you know, honestly, I don’t want to get in it.”

On the campaign trail, Trump has repeatedly suggested that the president might not be Christian or that he might sympathize with Islamic State terrorists.

Such attacks have caused many black voters to turn sharply against Trump, offended that he would challenge the qualifications of the country’s first black president. As Trump has made an aggressive pitch to minority voters in the past few weeks, there has been a renewed debate over Trump’s prominent role in the birther moment.

On Tuesday, Carson — now a regular Trump surrogate — said on CNN that Trump could immediately improve his relationship with African American voters by apologizing for questioning the president’s place of birth.

“I think that would be a good idea, absolutely,” Carson said. “I suggest that on all sides.”

Aaron Blake and Anne Gearan contributed to this report.