Donald Trump’s choice for senior communications director of the National Security Council plagiarized as many as 50 sources for her bestselling 2012 book, at times tweaking a few words from passages lifted from news articles, columnists, research institutes, and Wikipedia, according to a CNN report.

This isn’t the first time Monica Crowley, a conservative radio host, former Fox News commentator, and longtime Trump supporter, has been accused of plagiarism. Nor is it the first time a plagiarism scandal has shaken politics in the United States and Europe.

But the outcomes of these accusations have ended differently for various politicians and government officials. For some, an accusation of plagiarism cut short their campaigns or careers. Others were able to move past these damaging reports. It seems the threads that tie them together, however, are the questions raised time and again: In politics, can phrases and passages be borrowed, and when must they be cited?

According to the report from CNN KFile, the network’s investigative team led by Andrew Kaczynski, Ms. Crowley plagiarized large sections of “What The (Bleep) Just Happened,” with the book containing no notes or bibliography. CNN found that in The New York Times bestseller critical of President Obama’s first term, Crowley lifted whole passages from other sources. In one example CNN highlights, Crowley lifted a description of Keynesian economics from the website Investopedia. According to CNN, sections of Crowley’s book are also repeatedly lifted from articles by National Review columnist Andrew McCarthy, a friend of Crowley’s. Other sources plagiarized include the Associated Press, The New York Times, Politico, The Wall Street Journal, the BBC, and Yahoo News.

Crowley and her publisher, Harper Collins, did not return CNN’s requests for comment. But the Trump transition team defended Crowley in a statement, indicating she will continue to serve in the administration come inauguration Jan. 20.

"Any attempt to discredit Monica is nothing more than a politically motivated attack that seeks to distract from the real issues facing this country,” the statement from a transition spokesperson said.

Crowley, an assistant and confidante to former President Richard Nixon from 1990 until his death in 1994, is set to replace Ben Rhodes, Obama’s senior national security advisor. Mr. Rhodes is the "single most influential voice shaping American foreign policy aside from POTUS himself," wrote The New York Times. The Daily Beast added Crowley’s predecessor had "a critical hand in shaping the administration’s global security policies and then selling those policies to the world.

Crowley was previously accused of plagiarism for a 1998 column that compared the Watergate scandal to the Clintons. The column appeared on an editorial page of the Wall Street Journal on the 25th anniversary of Mr. Nixon’s resignation (and was also printed on The Christian Science Monitor’s website).

Four days later, the Journal published an editor’s note that reads: "There are striking similarities in phraseology between 'The Day Richard Nixon Said Goodbye,' an editorial feature Monday by Monica Crowley, and a 1988 article by Paul Johnson in Commentary magazine ... Had we known of the parallels, we would not have published the article.” Slate columnist Timothy Noah was highly critical of Crowley.

But Crowley defended herself to The New York Times.

"There are clear similarities in the language. I have wracked my brain, and I can honestly tell you that I have not read Johnson's article,” said Crowley, as Slate's Mr. Noah wrote at the time. “I am not a plagiarist!”

While the Trump team said Crowley would remain a part of the administration, other plagiarism scandals have ended differently for different political figures. One of the most famous in recent history involved Vice President Joe Biden. In his 1987 bid for president, then-Sen. Biden was accused of lifting the “phrases, gestures, and Welsh syntax” from a commercial by British Labour Party member Neil Kinnock just four months prior. The allegations ended Biden’s campaign. He withdrew from the race after the New York Times’s Maureen Dowd showed the similarities between the commercial and Biden’s closing speech at the Iowa State Fair.

“But Mr. Biden's borrowing raises questions about how much a candidate can adapt someone else's language and thoughts, whether he remembers to give credit or not,” wrote Ms. Dowd. “'Speeches Are Not Copyrighted'”

In 2008, a senior White House official under President George W. Bush resigned after he admitted to copying large sections of a column he wrote for a newspaper in Fort Wayne, Indiana, CNN reported at the time. Timothy Goeglein, who was a special assistant to the president and deputy director of public liaison, admitted he copied paragraphs verbatim from author Jeffrey Hart.

Dr. Hart, founder of the Dartmouth Review, accepted Mr. Goeglein’s personal apology via email.

"I told him I was flattered he'd used it. It doesn't damage him in my estimation at all,” Hart told CNN. “I'm glad he spread the word."

Other politicians who have been caught up in plagiarism allegations include Kentucky Sen. Rand Paul, Republican presidential candidate and bestselling author Ben Carson, and Montana Sen. John Walsh. In 2014, The New York Times reported Mr. Walsh plagiarized at least one quarter of his master’s thesis for the United States Army War College. The report ended the Democratic senator’s campaign for reelection.

This summer, Melania Trump’s speech at the Republican National Convention in Cleveland included two passages that contained words and thoughts very similar to those from a 2008 speech delivered by Michelle Obama. As Max Lewontin reported for The Christian Science Monitor at the time, the plagiarism allegations renewed questions about whether it’s unethical in politics to borrow words, phrases, and sections from other speeches.

"In politics, it’s not clear that we have an agreed-upon code of conduct, so the ethics almost blend sort of directly into the politics," Michael Carroll, a law professor at American University, told the Monitor. "If the people don't have a problem with it, is there some other reason to think it's a problem?"

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But some academics responding on Twitter to the news about Crowley Sunday were disturbed at the scale of the plagiarism examples in her book.

“To me it shows that the author (or ghost writer) just didn’t care about avoiding the most common form of plagiarism: lifting passages from texts that informed your writing,” wrote Jay Rosen, a journalism professor at New York University, on his blog PressThink. “One or two of these would be a minor violation of publishing standards. The pattern Kaczynski uncovered is a different matter entirely.”