CNN political commentator Keith Boykin said Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE’s repeated mentions of “Barack and I” border on"being disrespectful" because the former vice president was referring to his ex-boss by his first name only.

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“I do think that Joe Biden should refer to former President Obama as ‘President Obama,’ not as ‘Barack.’ It borders on being disrespectful," Boykin told CNN's Erin Burnett. "Despite their close relationship, I don’t think it’s appropriate for him to continue doing that."

Obama and Biden have touted their close friendship in the past, but the former president has said he will not endorse any Democratic candidates, including his vice president, in the Democratic primary. Biden has led polls for the Democratic presidential nomination.

"In 2008 we had a candidate who ran for president and lost, served in the Obama administration, then ran for president later on and became the Democratic nominee. Now we’re in the 2020 cycle. We have another candidate who ran for president in 2008 and lost who served in the Obama administration and now wants to run for president," Boykin said. "It didn’t work the last time. It’s not going to work this time unless you have some other message to offer.”

Biden had a sizable lead in the polls after launching his campaign in April, but has fallen in recent days following the first Democratic debates in Miami on June 26 and June 27.