Trump backers say the president has strong opposition from his National Security Council because he chose foreign policy experts instead of his loyalists as top advisers in the beginning and the staff was full of Obama supporters.

Most Obama-era NSC “detailees” — the term for temporary agency transfers — remained as the new White House was rocked by destabilizing anti-Trump leaks to the liberal media.

“Obama clearly left the White House booby-trapped,” Michael Caputo, a campaign adviser, told The Washington Times.

The NSC workforce’s size and loyalties have come into sharp focus now that a former staffer has filed a nine-page whistleblower complaint against President Trump. The person accuses Mr. Trump of withholding military aid to force Kyiv to investigate the roles of then-Vice President Joseph R. Biden and his son Hunter in a Ukrainian gas firm run by a corrupt oligarch.

The complaint emboldened Democrats to open an impeachment probe led by the House Permanent Select Committee on Intelligence. Witness transcripts have revealed how the NSC staff worked against Trump political appointees. One former NSC analyst made claims of Ukrainian meddling against a Republican colleague, who flatly denies the accusation.

The whistleblower is a CIA analyst and Democrat who worked closely with Vice President Joseph R. Biden from 2015 to 2016 and returned to the agency in June 2017.

As Mr. Trump won the election, a National Defense University scholar wrote a paper advising the next administration on how to mold an NSC bureaucracy.

“The NSC staff is a place for grownups, committed to serve the Nation and the President,” the paper read. “Only careful vetting and objective criteria will ensure an NSC staff that is up to the task.”

The NSC staff ballooned under President Barack Obama from 100 to 236 today. About 80% came as detailees from other agencies such as the CIA, the State Department and the Pentagon.

“His stacking and expansion of the NSC should have been the first warning,” Mr. Caputo said. “If there is any honor left in our republic, whoever told the president and the Trump family it was just fine to keep Obama holdovers at the NSC should be tarred, feathered and paraded for all the world to see.”

Out of the loop

Then-White House Chief of Staff Reince Priebus in 2017 became convinced that NSC detailees were leaking, sometimes inaccurately, details of Mr. Trump’s meetings and phone calls with foreign leaders. But he could never find the culprits, a former aide said.

The aide said some of the NSC detailees who arrived that year had bashed the president on social media.

J.D. Gordon, the Trump campaign’s director of national security and a former Pentagon spokesman, is one of those who were shunned by the new administration. He got caught up in the Trump-Russia probe but was never accused of wrongdoing.

“Trump set his own whistleblower and investigation traps by allowing his campaign staff and surrogates to be near-universally kept out of [the] administration while letting Obama holdovers stay and hiring people who didn’t support him or MAGA policies in general,” Mr. Gordon told The Times.

Mr. Trump did pick a loyalist, retired Army Lt. Gen. Michael Flynn, as his first national security adviser.

Little did he know that the Obama Justice Department already had targeted Flynn. Obama holdovers in January 2017 used a never-prosecuted 1799 law, the Logan Act, as justification to send two FBI agents, including Trump critic Peter Strzok, to interview him.

The topic was an intercepted phone call with the Russian ambassador. Someone leaked the call contents to The Washington Post. Flynn abruptly resigned and subsequently pleaded guilty to lying to the agents.

Mr. Gordon said Flynn worked outside the campaign and mostly ignored its people during the transition.

Trump supporters watched with disappointment as Flynn recruited as his top Russia adviser Fiona Hill. She didn’t come from Republican circles but from the Brookings Institution, one of Mr. Trump’s constant critics on domestic and foreign policy.

Released House impeachment transcripts show that Ms. Hill didn’t like the style of Gordon Sondland, Mr. Trump’s personal pick for ambassador to the European Union. She also criticized acting White House Chief of Staff Mick Mulvaney.

Ms. Hill also cited iffy hearsay to testify against NSC staffer Kashyap “Kash” Patel, a former intelligence committee investigator for Rep. Devin Nunes, California Republican.

Mr. Nunes’ Republican majority on the House intelligence committee in 2018 exonerated Mr. Trump in the Russia election conspiracy probe. In the process, he upset Democrats by finding out that their party had funded the discredited dossier that Democratic operatives spread within the Obama Justice Department.

Mr. Patel, a lawyer who played a significant role in both findings, became a liberal press target.

Special counsel Robert Mueller came to the same conclusion as Mr. Nunes. Mr. Mueller’s report released in March said his 22-month probe didn’t establish a Trump-Russia conspiracy.

Ms. Hill depicted Mr. Patel as going outside his assigned NSC portfolio and creating a backdoor channel to the president on Ukraine.

“Let’s just say it’s a red flag when somebody who you barely know is involved on one of your policy issues and is clearly providing, you know, materials outside of the line that we don’t even know what those materials are,” she said, adding that she removed Mr. Patel from her email distribution list.

Ms. Hill said she had no firsthand knowledge to support her accusation.

Mr. Patel categorically denied Ms. Hill’s allegations. He said he has never communicated anything about Ukraine to the president.

“Any reporting to the contrary, and any testimony provided to Congress, is simply false, and any current or former staff who suggest I have raised or discussed Ukraine matters with President Trump, are similarly misinformed or spreading outright falsehoods,” Mr. Patel said in a statement to Axios news site.

The holdovers

Leaving Obama holdovers in place became somewhat of a theme administrationwide as senior Trump officials passed over campaign loyalists.

Defense Secretary James N. Mattis wanted Hillary Clinton supporters, such as Michele Flournoy and Anne Patterson, as top defense officials, but he was rebuffed by the White House.

Mr. Mattis was not part of the Republican national security framework. Even more remote was Mr. Trump’s pick for the other plum Cabinet post: former oilman Rex W. Tillerson as secretary of state.

The post-Flynn national security adviser was retired Army Lt. Gen. H.R. McMaster, a renowned military strategist and Iraq War commander but not a Trump loyalist. Another retired officer, Marine Corps Gen. John F. Kelly, fit the same modus operandi as the White House chief of staff.

Mr. Trump’s ambassador to the United Nations, Nikki Haley, a former Republican governor, writes in her new memoir that Mr. Tillerson and Mr. Kelly were not loyal to the Trump agenda, The Washington Post reported.

In contrast, Mr. Obama made sure his chiefs of staff — Rahm Emanuel, William Daley, Jack Lew and Denis McDonough — came from pure Democratic Party stock. Mr. McDonough, his longest-serving and final chief, had worked for Senate Democratic leader Tom Daschle and for the liberal Center for American Progress founded by Clinton ally John Podesta.

Another Trump critic on the NSC staff is Army Lt. Col. Alexander Vindman, who arrived from the Joint Chiefs of Staff planning apparatus in 2018.

Col. Vindman immediately filed a complaint against his commander in chief with the NSC counsel after listening in on Mr. Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky.

He also testified that he resented two U.S. European ambassadors’ injections into his interagency discussions on Ukraine policy.

Created by federal law in 1947, the NSC has expanded its staff and influence. President Eisenhower relied on a staff of fewer than 20. Today, it numbers 236.

Those NSC salad days may be coming to an end.

National Security Adviser Robert C. O’Brien, who replaced John R. Bolton, said on CBS’s “Face the Nation” that he is streamlining a “bloated” organization.

“So everyone who’s detailed at the NSC, people are going to start going back to their own departments, and we’ll bring in new folks,” Mr. O’Brien said. “But we’re going to get that number down to around 100 people. … We don’t need to re-create the Department of Defense, the Department of State, the Department of Homeland Security over at the White House. We’ve got great diplomats and soldiers and folks that can … do that work for us in the departments.”

After Mr. Trump won the election, National Defense University’s R.D. Hooker Jr. wrote some advice about what kind of NSC workforce the president should assemble.

“In general, the President is best served when NSC staff members are carefully screened for their intelligence, academic and professional qualifications, experience, and willingness to serve out of the limelight,” wrote Mr. Hooker, an NSC alum. “Excessive ambition, personal agendas, and divided loyalties can only distract and disrupt complex and highly sensitive NSC machinery. The NSC staff is a place for grownups, committed to serve the Nation and the President. Only careful vetting and objective criteria will ensure an NSC staff that is up to the task.”