Clip_10 (2)Among the trove of emails released from Hillary Rodham Clinton’s tenure as secretary of state was this instruction to a trusted aide who needed to brief her on a matter that could not wait:

“Just knock on the door to the bedroom if it’s closed,” Mrs. Clinton wrote in November 2009 to Huma Abedin, then her deputy chief of staff.

Since she first interned for Mrs. Clinton as a 19-year-old George Washington University student, Ms. Abedin, 40, has held a singular role as one of the former first lady’s most trusted and most visible confidantes. She travels with Mrs. Clinton to her presidential campaign events and fund-raisers, dispensing political advice and serving as a gatekeeper for donors, Democratic leaders and even supporters who want a photograph with the candidate.

As questions continue to dog Mrs. Clinton about her use of a private email account, a spotlight has landed on Ms. Abedin, the aide so often at her side that she has been called Mrs. Clinton’s “surrogate daughter.”

Ms. Abedin’s own emails on her boss’s private server have drawn increasingly intense scrutiny — as has an arrangement she made to earn income privately while she worked for Mrs. Clinton at the State Department. Ms. Abedin was on Mrs. Clinton’s personal payroll, and her other outside employers were the Clinton Foundation and Teneo, a consulting firm founded in part by Douglas J. Band, who was a counselor to former President Bill Clinton.

When that arrangement was revealed more than two years ago, political opponents including Senator Charles E. Grassley, Republican of Iowa and chairman of the Judiciary Committee, raised concerns about potential conflicts of interest. But those quieted until the email controversy provided a new opening — and the potential for new information to be unearthed.

HumaIn a recent letter to the State Department, Mr. Grassley suggested that Ms. Abedin, at Mr. Band’s request, may have asked Mrs. Clinton to urge President Obama to give a White House appointment to a Teneo client, Judith Rodin, the head of the Rockefeller Foundation.

Conservative groups like Judicial Watch and Citizens United have also applied pressure through Freedom of Information Act lawsuits and other measures to press the State Department to make public Ms. Abedin’s emails and employment records.

Those records, they argue, could give clarity about Ms. Abedin’s outside work and whether, as critics contend, it converged with her official duties. (Ms. Abedin’s lawyers say she has cooperated with all requests made in the lawsuits.)

Ms. Abedin declined to comment for this article. But in a letter sent Friday to the legal adviser’s office of the State Department, her lawyer, Miguel E. Rodriguez, wrote that Mr. Grassley’s notion that Ms. Abedin may have intervened on Ms. Rodin’s behalf was demonstrably false — Ms. Rodin received a presidential appointment two years before Ms. Abedin began working for Teneo — and that Mr. Grassley was citing hearsay related to email exchanges he had not seen.

Mr. Grassley, the lawyer wrote, had “unfairly tarnished Ms. Abedin’s reputation by making unsubstantiated allegations.”

Mrs. Clinton’s critics have also seized on an inquiry by the State Department’s inspector general into whether Ms. Abedin was overpaid by the government by nearly $10,000 when she was on maternity leave in 2012. But Mr. Rodriguez has disputed those findings, saying the inspector general’s own report showed that Ms. Abedin was working while she was on maternity leave and thus had earned the extra pay.

Ms. Abedin, at a campaign stop in Norwalk, Iowa, in April. She serves as a gatekeeper for donors, Democratic leaders and supporters of Mrs. Clinton.CreditDoug Mills/The New York Times

In the letter Friday, Mr. Rodriguez wrote that Ms. Abedin’s “integrity and her tireless work ethic” made her “a role model for young women.”

Mrs. Clinton has been unable to put to rest questions about her email as some of her poll numbers have weakened and a new threat has emerged with Vice President Joseph R. Biden Jr. possibly joining the Democratic primary field. Now her campaign is mounting an aggressive defense of Ms. Abedin.

“Huma is wicked smart, multifaceted and has a great strategic sense,” the campaign’s chairman, John Podesta, said in a statement in which he also praised her humility. “She’s an integral part of the team, and we’re not going to tolerate these attempts to turn piecemeal, partisan leaks into an alternative reality. If the Republicans on Capitol Hill think they can bully her, they are dealing with the wrong woman.”

Clinton allies say Ms. Abedin is merely a pawn in congressional Republicans’ efforts to damage Mrs. Clinton, but one that will ultimately provide little fodder. And they say that Ms. Abedin, who is Muslim and was raised in Saudi Arabia, has long inspired conspiracy theories among conservatives eager to sully Mrs. Clinton.

But the email controversy and the outsize attention Ms. Abedin has attracted pose challenges as her role in the campaign evolves.

Ms. Abedin has long since graduated from the so-called body woman’s work of fetching Diet Cokes or the jalapeño peppers Mrs. Clinton eats to maintain her voice on the road. She helped plot Mrs. Clinton’s political activity during the 2014 midterm elections, orchestrated the promotion of her 2014 memoir, “Hard Choices,” and was rewarded by being named the vice chairwoman of her presidential campaign.

Though she is routinely at Mrs. Clinton’s side on the road, advising on policy and politics, Ms. Abedin — known as genial and easygoing, and for gushing about her young son, Jordan — has a corner office in the campaign’s Brooklyn headquarters with a sign marked for the vice chairwoman. She oversees scheduling, correspondence, surrogates, briefings and the advance staff members who set up events nationwide. She has also stood in for Mrs. Clinton at some small fund-raisers.

“Now, politically, organizationally and personally, she’s the glue that holds Clinton World together,” said Robert Barnett, a Washington lawyer who has worked with the Clintons for years. “She has the complete and unquestioned trust of the secretary.”

The ties between Mrs. Clinton and Ms. Abedin, called “Hillary’s Secret Weapon” by Vogue magazine in 2007, have only grown stronger since.

Mr. Clinton officiated at her 2010 wedding to Representative Anthony D. Weiner of New York, at Oheka Castle in Huntington, N.Y., just weeks before Chelsea Clinton’s wedding. A year later, when Mr. Weiner resigned from Congress amid a sexting scandal, Ms. Abedin’s decision to stand by him evoked comparisons to the marital travails her boss faced in the 1990s.

When Mr. Weiner tried for a political comeback in the 2013 New York mayor’s race, and new revelations showed he had continued his sexting ways even after leaving Congress, people who wanted to protect Mrs. Clinton’s image as she contemplated a 2016 White House run were not shy about urging him to drop out. (Mr. Weiner recently took a job at a communications firm owned by a Clinton donor.)

It was after Mr. Weiner’s resignation from Congress that Mr. Band, a friend of Ms. Abedin’s since their time working at the White House, offered her work at Teneo. He told people at the time that he hoped it would open her eyes to the possibilities outside the Clintons’ orbit. Ms. Abedin handled discrete projects, like a Teneo annual event, and did not influence State Department business, people briefed on her work at the time said.

But Ms. Abedin eventually decided consulting was not for her.

Marc Lasry, a Clinton donor and friend who attended Ms. Abedin’s wedding, called her “as loyal as they come.”

“She loves Hillary. She would die for Hillary,” Mr. Lasry said. “Really, what she’s always looking at is, ‘What’s in the best interest of Hillary?’ ”

Mr. Lasry said he had expected Ms. Abedin to reduce her workload in the past few years. Instead, she has maintained a breakneck pace.

“Her life,” he said, “is Hillary.”