Hillary Clinton says she would not "rule out" questioning the legitimacy of the US presidential election if Russia is found to have deeply meddled.
The former secretary of state told NPR's Fresh Air programme, however, she did not believe there was a "mechanism" in place to challenge the outcome.
US intelligence agencies believe Russia tried to tip the election in favour of President Donald Trump.
Mrs Clinton spoke just before her top aide testified to Congress.
John Podesta, the former chairman of her Democratic presidential campaign, appeared on Monday before the Senate Intelligence Committee, which is leading one of several investigations on Russia's alleged role in the 2016 election.
Mr Podesta was one of several Democratic figures whose email account was breached by suspected Russia-backed hackers.
The ghost of elections past?
Analysis by Anthony Zurcher, BBC North America reporter
The day after her 2016 election defeat, Hillary Clinton told a crowd of shell-shocked supporters that "we must accept this result and then look to the future".
Now, the former Democratic nominee seems open to adding a big asterisk to that sentence. Everyone can move on - unless there's clear evidence of Russian interference.
Needless to say, that's a big "unless" and a bridge that will likely never be crossed in the minds of many Americans.
It could come as sweet music to some in the Democratic base, who are hanging on every unconfirmed report or rumour of Trump campaign malfeasance and waiting with bated breath for the day the presidential house of cards comes crashing down.
Democratic officeholders and those aspiring to higher office - including the growing list of 2020 presidential aspirants - aren't quite so thrilled.
For them, Mrs Clinton is threatening to become the ghost of elections past; an unwelcome spectre reminding the party of its failings.
Mrs Clinton's recent statement - indeed, the tone of her book tour and the book itself - shows that while many Democrats may want to look to the future, she's still got one eye on what could have been.
His emails were subsequently released to the public by Wikileaks during the election campaign.
NPR's Terry Gross pressed Mrs Clinton on Monday about whether she would "completely rule out questioning the legitimacy of this election if we learn that the Russian interference in the election is even deeper than we know now".
"No, I wouldn't rule it out," Mrs Clinton said in the interview, which was to promote her new book about how she lost, What Happened.
But she emphasised that she did not believe there was a legal option to challenge the validity of the election.
"There are scholars, academics, who have arguments that it would be, but I don't think they're on strong ground. But people are making those arguments. I just don't think we have a mechanism," she said.
Republican Senator Cory Gardner fired back at Mrs Clinton on Monday, saying he is aware she has a book to plug but "she should be ashamed" for her "wildly irresponsible" comments.
Last week, Mrs Clinton called for the abolishing of the US electoral college as she sought to explain why she lost the election.
"I think it needs to be eliminated," Mrs Clinton told CNN of the institution. "I'd like to see us move beyond it, yes."
I know Secretary Clinton is trying to sell books but this is wildly irresponsible and she should be ashamed. https://t.co/TL5KYhfwsn— Cory Gardner (@SenCoryGardner) September 18, 2017
As Mrs Clinton often points out, she won the popular vote last November by nearly three million ballots.
But she still lost the White House because Mr Trump prevailed in the electoral college.
Despite winning the presidency, Mr Trump has himself questioned the results.
He has claimed millions voted illegally for Mrs Clinton, even though the Federal Election Commission certified the results.
The Trump administration has been dogged by claims that members of its campaign team had ties to the Kremlin.
Mr Trump denies any wrongdoing and has repeatedly called the Russia investigation a "witch hunt".