Two years ago, Mellencamp chided Walker, who effectively ended collective bargaining for most public workers in Wisconsin, for playing his song "Small Town" at campaign rallies.
Walker was targeted for recall in 2012 by those angered over the union law, which passed despite massive protests that elicited support from other liberal musicians, including Pete Seeger, Rage Against The Machine guitarist Tom Morello, Peter Yarrow and Billy Bragg.
When Mellencamp caught wind of Walker playing "Small Town" during the recall campaign, his publicist sent Walker an email letting him know that Mellencamp supports collective bargaining and union rights.
Mellencamp took the same approach in 2008 when Republican presidential hopeful John McCain played "Our Country" on the campaign trail. Mellencamp's publicist wrote McCain's camp a letter explaining Mellencamp's liberal leanings and said he was supporting Democrat John Edwards.
In 2010, when the National Organization for Marriage played "Pink Houses" at events opposing same-sex marriage, Mellencamp also had his publicist notify the group saying he was opposed to its agenda and suggested it pick a different song to play that is more in tune with its views.
When Walker launched his re-election campaign with a series of rallies Tuesday, one of the songs played while his supporters waited for the governor was "Pink Houses." The song, first released in 1983, contains the lyrics, "Ain't that America, home of the free. Little pink houses for you and me."
Mellencamp has not directly contacted Walker's campaign this year about his use of the song, said the rocker's publicist Claire Mercuri. But Mellencamp wants to again let Walker know about his support of unions and collective bargaining, Mercuri said.
"Nothing has changed since the last time Gov. Walker ran for election," Mercuri said in an email to The Associated Press.
Walker told AP he's been a Mellencamp fan for years, and said he'll continue to play the artist's songs at campaign events.
"You don't have to agree with his politics to like his music," Walker said.
Mellencamp himself played "Pink Houses" at President Barack Obama's inauguration celebration in January 2009. Edwards, the former Democratic senator who ran for president in 2004 and 2008, also used the song with no interference from Mellencamp.