Hillary Clinton just had her toughest grilling yet

Former Secretary of State Hillary Clinton's Sunday "Meet the Press" interview featured question after question on her email use at the State Department.

Host Chuck Todd asked the Democratic presidential front-runner roughly a dozen inquiries about the controversy - before moving on to questions about her sinking poll numbers and policy flip-flops over the years.

Todd began the interview by playing a seven-year-old clip of Clinton saying she wants "a much more transparent government."

He suggested that it might have been hypocritical for Clinton to exclusively use a private email account while she was at the State Department - placing her messages outside of government archives.

But Clinton argued that she actually went out of her way to include government email addresses in her work-related communications. 

"All of the emails that I sent were intended to be in the government system if they were work-related," she said. "If I had to do it all over again, I would have used a separate email account. I did it for convenience and it turned out not to be that at all."

Clinton also said her team conducted a "careful review" and turned over all of her work-related emails over to government archivists. But Todd then pressed Clinton twice about a report last week that she failed to turn over an email chain with former then-commander of US Central Command David Petraeus, which challenged the completeness of her records.

"The reason that we know about the email chain with Gen. Petraeus is because it was on a government server," Clinton responded without offering a specific explanation for the discrepancy. "We had a very thorough review process that we conducted. And my attorney supervised it. They went through everything."

Todd went on to press Clinton on the elaborate amount of work needed to set up a personal email server, how practical it was to rely on recipients' government email addresses for archiving purposes, and a recent Washington Post report on the differences between how she and the State Department describe the department's initial request that she turn over her emails. 

"Perhaps the reason you wanted to have a private server and not a government server is that Republicans have been coming after you for years, you might have been running for president in the future, and you wanted to make it a little more difficult for congressional investigators to subpoena your government emails and a little more difficult for Freedom of Information Act requests. Is that a fair theory or not?" Todd asked.

Clinton called Todd's theory "totally ridiculous" and that it "never crossed my mind."

Clinton's presidential campaign has been battered for months by critical headlines about her email use and her decision to delete the entire email trove after turning over the messages deemed work-related to the government.

Clinton insists she did not jeopardize sensitive information, for which she said she used secure channels. But the FBI reportedly took control of the server to look into whether any material was mishandled in connection to the account. 

And as Todd noted, the story doesn't seem to be going away.

"There's an allegation about your email server, the campaign provides an explanation, you provide an explanation, there's a new allegation, you have to provide a new explanation, there's an addendum to that explanation. It has the feel of a 'drip, drip, drip.' Can you reassure Democrats that there's nothing else here?" Todd asked.

Clinton agreed but said there is only so much she could do to reassure people concerned about the evolving story line.

"Well, it is like a drip, drip, drip," she said. "I want these questions to be answered. I can't predict to you what the Republicans will come up with, what kind charges or claims they might make. I have no control over that. I can only do the best I can to try to respond." 

Todd responded by asking if she could "say with 100% certainty that the deleted emails that the FBI's not going to find anything in there that's going to cause you to have to explain again?"

Clinton suggested she couldn't be that certain.

"All I can tell you is that when my attorneys conducted this exhaustive process, I did not participate," she said.

Later in the interview, Todd asked Clinton how she treated emails related to her family foundation and and how the controversy damaged her standing in the polls. Sen. Bernie Sanders (I-Vermont), running as a Democrat, has surged in early-state surveys, while Vice President Joe Biden, who is still weighing run, already has a solid base of support.

Todd also played a montage of Clinton's flip-flops on issues over the years - including her vote on the 2003 invasion of Iraq that she later admitted was a mistake, her past opposition to same-sex marriage, and her statement that she was "inclined" to support the Keystone XL Pipeline to which she recently announced her opposition.

"I can just tell you that I am not someone who stakes out a position and holds it regardless of the evidence," Clinton said after offering a point-by-point defense of her policy shifts.

Watch parts of the interview below: