"Her strengths, which are the fact that she's extraordinarily experienced and wicked smart, and knows every policy inside and out, sometimes could make her more cautious and her campaign more prose than poetry."
The President added: "You're going to dig into his [Sanders'] proposals and how much they cost and what does it mean, and how does his tax policy work, and he's subjected then to a rigour that hasn't happened yet."
Asked if Sanders' insurgent campaign reminded him of his own victory against Clinton in 2008, Obama said: "I don't think that's true."
An aide to the President said: "He's not panicked by Sanders but he's clearly thumbing the scale for Hillary."
Clinton has been facing relentless attacks from Republicans over her use of a private email server when she was Secretary of State and her handling of the 2012 terrorist attack in Benghazi in which the US ambassador to Libya, Chris Stevens, was killed.
Last week she was criticised by some of her own supporters after she spoke for only five minutes at a rally in Iowa with the pop star Demi Lovato.
Clinton's aides have said privately that they may have underestimated Sanders and ignored him for too long.
A week before the Iowa vote, Clinton has begun attacking Sanders on issues including his lack of foreign policy experience, pro-gun stance and how he would pay for his health and education policies.
Her supporters have suggested Sanders would lose in a landslide to the Republicans.
Obama says
On Clinton: Her strengths, which are the fact that she's extraordinarily experienced and wicked smart, and knows every policy inside and out, sometimes could make her more cautious and her campaign more prose than poetry.
On Sanders: You're going to dig into his proposals and how much they cost and what does it mean, and how does his tax policy work, and he's subjected then to a rigour that hasn't happened yet.
