Sorry, bad idea.  Collin Peterson represents his district and the positions popular there quite well.  (Unlike, say, Jim Moran, who should be much further to the left than he is given his constituency.)

If Peterson ever loses a Democratic primary to somebody on the left, we lose that seat to a republican.  That's almost a given.  Plus, Peterson has a good AFL-CIO record, decent on environmental issues, and is pro-gun rights (an issue the Dems desparately need to change on).  Of a list of Dems I'd like to see challenged from the left, Peterson doesn't even make the top 100.