(YAHOO) - Barack Obama reportedly spoke with a Democrat, expressing his private doubts about Joe Biden becoming the Democrat presidential nominee in 2020.

According to Politico, the former president said: "Don't underestimate Joe's ability to f... things up."

Mr. Obama was also said to have spoken about his own understanding of the 2020 Democrat electorate in Iowa.

"And you know who really doesn’t have it? Joe Biden," he reportedly said.

Apparently, there were lingering tensions between the two after Mr. Obama supported Hillary Clinton for the 2016 nomination, according to Yahoo.   

Leon Panetta, Mr. Obama’s defense secretary, told Politico: "He [Mr Biden] was loyal, I think, to Obama in every way in terms of defending and standing by him, even probably when he disagreed with what Obama was doing.

"To some extent, [he] oftentimes felt that that loyalty was not being rewarded.”

Several of Mr Obama's aides were said to have believed that Mr. Biden took discussions in the wrong direction while vice president.

In his book Ben Rhodes,  Obama’s former deputy national security adviser, penned, "in the Situation Room, Biden could be something of an unguided missile”.

Mr. Biden's aides were said to have believed his skills getting things achieved in Congress were under-appreciated.

Since Mr. Biden secured the Democrat nomination Mr Obama has enthusiastically backed him and will be a keynote speaker at the Democrat Convention next week.