On the campaign trail, Democratic presidential contender Pete Buttigieg blasts Vice President Mike Pence's cultural and religious conservatism. But as the mayor of Indiana's fourth largest city, his tone toward the state's Republican former governor was more muted.

During the four years in which they overlapped in Indiana politics, Buttigieg, the South Bend mayor, had a cordial relationship with Pence. The two collaborated on economic development issues . Buttigieg presented Pence with a South Bend promotional T-shirt that said "I (heart) SB." And at ceremonial events, Pence would lavish Buttigieg with praise.

The relationship between the two men has come under scrutiny as Buttigieg's campaign becomes a surprise hit, raking in $7 million during the first quarter. As he formally launches his White House campaign on Sunday, the gay mayorcampaign memoir, his relationship with Pence is "complicated." In an interview with CNBC that aired on Thursday, Pence said they had a "great working relationship" and criticized the mayor's characterization of his religious beliefs.

"He knows better," Pence said. "He knows me."

The complex dynamic between Buttigieg and Pence reflects a connection born of political necessity between two men on the cusp of unlikely political and personal paths. And to some fellow Indiana Democrats, it was frustrating.

In a state with few influential Democrats, some hoped to tap into Buttigieg's burgeoning political celebrity to help them rebut Pence and his policies, according to two party strategists who spoke on the condition of anonymity to discuss a sensitive internal debate. But they found he was often reluctant to take on the then-governor