Now that's who then Vice President Gore should have selected as his running mate in 2000. However we cannot blame the selection of Senator Lieberman as the prime cause of Gore's defeat in 2000. His Presidential campaign for starters was lacklustre, mediocre at most, etc.
As for Senator Kerry's Vice Presidential selection in 2004, that would have ensured him victory over President Bush, I would say Congressman Dick Gephardt of Missouri. I believe he could have pulled in Iowa, Missouri and West Virginia into the Kerry column, considering his strong connections with Labor groups. However that is a very outlandish statement to make, regarding Gephardt's ability to swing as many as three states.