The leader of the Democratic National Committee (DNC) must be able to do the job full-time, Howard Dean said Monday, casting doubt on the viability of Rep. Keith Ellison (D-Minn.), who is reportedly interested in the position.

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Dean, who is running to be DNC chairman, said he respects Ellison, but it is difficult to be both a congressman and chairman of the party.

"I think Keith is terrific. I went door to door with him on his first campaign. I like him a lot. You cannot do this job if it's not full-time. Period. Full stop. End of story," Dean said on MSNBC's "Morning Joe." 

Dean, who served as the DNC chairman from 2005 to 2009, said he will still support the winner. 

Democrats in Washington appear to be lining up behind Ellison, the first Muslim elected to Congress. 

Sen. Bernie SandersBernie SandersFive takeaways from the Democratic debate As Buttigieg rises, Biden is still the target Leading Democrats largely pull punches at debate MORE (I-Vt.) and retiring Senate Minority Leader Harry ReidHarry Mason ReidHarry Reid: Early voting states Iowa, New Hampshire 'not representative of the country anymore' The Memo: Democrats confront prospect of long primary Bottom Line MORE (D-Nev.) have both endorsed Ellison to be the next DNC chairman.

“My friend Keith Ellison is a terrific leader and a strong progressive who knows how to get things done,” Reid said.

“Now is the time for new thinking and a fresh start at the DNC. Now is the time for Keith.”