"To some extent, I kind of feel like Secretary Clinton is a leaky boat at this point and that concerns me as a Democrat," Aiken said.
Aiken, who won the hearts of millions of Americans in 2003 when he placed second on "American Idol," backed Clinton in the 2008 Democratic primary and expressed his support when Clinton declared her second presidential bid in April.
But as questions about Clinton's trustworthiness loomed, largely in light of controversy around her exclusive use of personal email while at the State Department, Aiken had a change of heart.
Aiken told CNN that while he has "nothing but respect" for Clinton," he worries that "a lot of (moderates) have negative feelings towards (her)."
"I'll get on a leaky boat going in the right direction before I'll get on a strong boat going in the wrong direction," Aiken said, "But I really want a strong boat next year on the Democratic side and I think Joe Biden is a better candidate."
Biden gaffes: 'moments of authenticity'
Biden is known for off-the-cuff colorful moments
-- some of which have caused headaches for the Obama administration. After introducing President Barack Obama during the Obamacare signing ceremony in 2010, Biden turned to Obama and said, "This is a big f------ deal."
When asked about the famous and at times infamous Biden gaffes, Aiken said that those moments should not be referred to as gaffes and are, in fact, "moments of authenticity" that voters respond to positively.
"I think there is something refreshing about it and I think people are starting to recognize that," Aiken said
Aiken previously called Clinton "stiff" and advised her to "loosen up."
"@HillaryClinton, I love you. But why are you the stiffest and most scripted looking person in this video? #loosenup," Aiken tweeted.
Aiken said such moments of authenticity seem to be resonating with voters and making candidates more likable.
"If you look at the person who has said more things that would be considered gaffes than anybody else -- Donald Trump -- seriously, and he's at the top of the pack," Aiken said. "I don't think gaffes are a liability. I don't think they should be called gaffes."
Clinton, who recently appeared on NBC's "Saturday Night Live" has sought to become more spontaneous as her campaign moves forward.
'Big mistake' not to take Trump seriously
Aiken, who is also a former competitor on NBC's "The Apprentice" knows Trump personally and was actually fired
by him on the show. (He came in second to Arsenio Hall in 2012.)
"I'm a friend of his. I like him as a person. He's always been very kind to me except for when he made me lose on 'The Apprentice,'" Aiken joked of Trump. "But he's a good guy and I certainly think people are mistaken if they don't take him seriously and we see that in the poll numbers."
Aiken likens Trump to "that uncle, who embarrasses the hell out of you sometimes and you still love them, but damn, you wish they'd shut up."
But for Aiken, the "biggest testament" to Trump being a nice guy are his children.
Ivanka, Eric and Donald Jr. "are some of the most grounded, smart, un-entitled individuals that you'd ever care to meet," Aiken said. "They are incredibly respectful and smart and driven and grounded and I think that's a testament to somebody as a father."
Why he ran for office
Aiken said that becoming a father is one of the reasons that prompted him to run for public office. He lost
his congressional bid to represent North Carolina's 2nd District to Republican Rep. Renee Ellmers in 2014.
"I didn't run because I wanted to be a politician. I ran because I saw a need. I think that need still exists," Aiken said, naming education as one of his top issues.
He said he was disappointed that the National Education Association, the nation's largest teachers union has already endorsed
"As a member of the NEA, I would have hoped that they had been a little bit more patient to see, first ... to really let us hear the positions of everyone before they endorsed," Aiken said.
But he added that none of the candidates on either side have talked about education enough to make him happy.
Aiken is an admirer of southern politicians willing to "buck the trends in the South" and cited former North Carolina Gov. Terry Sanford as his political idol.
"He was the first and only governor in the South who spoke up against segregation so vocally and he consolidated the university system in North Carolina and really understood the importance of education to improving the economy," Aiken said.
But Aiken won't be running again "anytime soon."
"I don't think I'll run in 2016 -- that's for sure. Probably not in 2018, maybe not even in 2020. I think eventually, I'll run again," Aiken said.