WASHINGTON >> It’s been called the worst job in Washington. The gatekeeper to the most powerful leader on earth. The president’s alter ego or the chief javelin catcher.

The job of White House chief of staff is at the fulcrum of the federal government, yet it’s a role that remains largely opaque outside of Washington circles. The newest person to assume the title is Jeff Zients, a longtime Washington hand with a reputation as a managerial whiz who became President Joe Biden’s second chief of staff last week.

Sen. Mitt Romney, R-Utah, said the White House chief of staff is a kind of a “chief operating officer of the country.”

“He’s not the biggest problem solver. He shouldn’t have to do the analysis and he’s got all sorts of other people that will do that,” said Romney, who pondered his own chief of staff picks when he was the GOP presidential nominee in 2012. “But he’s got to run the government, and that’s a task which very few chiefs of staff have had real experience in doing.”

So what does a White House chief of staff actually do?

Zients is literally the chief of the West Wing staff, ensuring that trains run on time and that the president is well served by aides. Zients is the one who presents options to the president on any number of executive decisions.

The chief of staff controls access to the president and is charged with turning the administration’s ambitions into reality. The president’s top aide is part of the Cabinet and must maintain good relationships with the heads of other agencies to ensure they are all on the same page. The job means juggling countless competing constituencies and often being the person who has to say “no” to them.

In a 2005 Washington Post article, Andy Card, who served President George W. Bush for nearly six years, likened his approach to managing a kitchen. Top priority items were on the front and back burners of the stove and longer-term tasks got stashed into the freezer.

“I’ve described it as a wind tunnel,” said Mack McLarty, who was President Bill Clinton’s first chief of staff. McLarty recalled that as he was preparing to assume the role, he was told by Howard Baker, chief of staff in President Ronald Reagan’s second term, that there was no worse job in the nation’s capital.

During his first 10 days on the job, Zients has had to handle long-planned White House priorities ( a planned trip to Poland to mark one year since Russia invaded Ukraine) and unexpected challenges ( multiple unidentified objects shot down from the sky). All throughout, Zients’ chief objective as the president prepares for a likely reelection campaign is to seamlessly implement several landmark bills that Biden signed into law in his first two years.

“As a team, our approach to delivering results for the American people will be straightforward,” Zients wrote in a note to White House aides on his first day as chief of staff. “We must aggressively and equitably implement the President’s policies, ensure that Americans know how to access these benefits and clearly communicate what we have accomplished on behalf of ALL Americans.”

The new chief of staff, who was an initial investor in a Washington bagel shop, immediately resurrected an old tradition from his previous stint in the White House: Bagel Wednesdays.

Those who have been a White House chief of staff, as well as those who have studied them, can point to several traits that are key to success: experience serving in previous administrations, an intimate familiarity with Capitol Hill, managerial acumen and political shrewdness. Also: a temperament that doesn’t gyrate with the whims of a news cycle — not to mention a close, personal relationship with the president.

Past chiefs of staff with that precise collection of characteristics, according to Chris Whipple, who wrote extensively on the role for “The Gatekeepers,” include James Baker, Reagan’s first chief of staff; Leon Panetta, Clinton’s second chief of staff; and Ron Klain, who recently exited the White House after serving as Biden’s chief of staff his first two years.

“Every president learns — sometimes the hard way — that he cannot govern effectively without empowering a White House chief of staff as first among equals in the West Wing to execute his agenda and to tell him what he doesn’t want to hear,” said Whipple, who spent extensive time with Klain for his latest book, “The Fight of His Life.”

Panetta, who would go on to serve as CIA director and defense secretary under President Barack Obama, agreed.

“I think the success or failure of any chief of staff is going to be very dependent on the relationship that that individual has with the president,” Panetta said in an interview. “In order for any chief of staff to do his job, he absolutely has to have the trust of the president of the United States, and the two of them have to be able to trust one another.”

Romney said he would have picked either Mike Leavitt, a former Utah governor and Bush’s health secretary who led Romney’s would-be transition team, or just-retired Sen. Rob Portman, R-Ohio, as chief of staff had Romney won the presidency. He said both possessed “extraordinary management capacity.”

Panetta also underscored the importance of the White House chief of staff maintaining close ties on Capitol Hill, which steers so much of the administration’s objectives or impede them. Zients has never worked in Congress, although he has won praise from Republican lawmakers from his time leading the Biden White House’s COVID-19 response team.

Lawmakers “can set the tone for what’s before the White House for that particular day, as to whether they’re having a good day or a bad day,” Panetta said.

As for those who weren’t quite as successful in the role, Whipple pointed to Mark Meadows, President Donald Trump’s final chief of staff — noting that Meadows declined to keep tight controls on who had access to Trump and did not push back on the president’s most outlandish demands.

“He swung the gate wide open for this Star Wars bar cast of characters parading into the Oval from Rudy Giuliani to Sidney Powell to the pillow guy,” said Whipple, referring to MyPillow chief executive Mike Lindell, a high-profile Trump ally. Giuliani was Trump’s personal lawyer and Powell was a lawyer for Trump’s campaign’s legal team who was booted from that role after pushing unfounded conspiracy theories after the 2020 election.

Whipple also said Donald Regan, Baker’s successor under Reagan, did not serve the president well as the Iran-contra affair engulfed the Reagan administration, and saw himself akin to a prime minister with little interest in the “staff” part of his title. Regan, who died in 2003, famously feuded with first lady Nancy Reagan, who helped force him out in 1987.

In picking a chief of staff, presidents sometimes gravitate toward someone with whom they’ve had long-held ties. McLarty was a kindergarten classmate of Clinton in Hope, Arkansas. Klain spent decades alongside Biden on Capitol Hill and was a campaign adviser on Biden’s previous presidential bids and chief of staff when Biden was vice president.

Trump went the opposite direction with his first chief of staff, picking Reince Priebus, who led the Republican National Committee in 2016. Priebus struggled to unite the warring factions within the White House. He never overcame deep suspicion from Trump loyalists who viewed him as a stalwart of the party establishment they reviled. Priebus was unceremoniously fired via tweet just six months into Trump’s term.

Biden signaled the importance he places on his chief of staff during an event honoring Klain, saying a president is “only as good as the team you put around you.”

The White House chief of staff is one of just a handful of the most powerful jobs in government that have been occupied only by white men. Asked last month whether Biden would select someone who is not white or male if he has the chance to choose another chief of staff after Zients, White House press secretary Karine Jean-Pierre declined to say.

But she said the current administration is “the most diverse in history” and “we expect this trend to continue.”