"His record has been distorted," Cleland said on "CBS This Morning," adding that he's "been shot on the battlefield, [and] he's been shot at on the battlefield in Washington."
"They are swatting at nothing," Cleland said of Hagel's critics.
Cleland, who served with Hagel in Vietnam and in the Senate, said Hagel "doesn't have to modify" any of his past statements but "has to clarify."
Hagel has come under scrutiny by some Republicans, gay organizations and pro-Israeli groups for some the statements and positions he's taken as a Senator from 1997 to 2009.
As CBS News' Bill Plante reports, Sen. John McCain, R-Ariz., said he has "serious concerns" about some of Hagel's previous stances, including a past position in support of direct talks over unilateral sanctions with Iran.
President Obama said Monday that he admires Hagel's "courage and his judgment, his willingness to speak his mind even if it wasn't popular."
Hagel defended himself, insisting that he receive "a full, open, transparent hearing."
Cleland said a Senate confirmation hearing is the "proper forum" for Hagel to explain his positions and to be fully scrutinized.
Clelend was confident that Hagel would prove his critics wrong. "[T]here's not doubt" that the Senate will back Hagel.
"Chuck Hagel is his own man. He wears no man's collar.... That's the kind of person you want" as defense secretary, Cleland said.