Because it tempts people to believing that the next party standard-bearer is not a fallible human being, but actually their Savior.
Q.v., this story from the Atlanta Journal-Constitution:
Georgia Republican Party chairwoman Sue Everhart said Saturday that the party’s presumed presidential nominee has a lot in common with Jesus Christ.
“John McCain is kind of like Jesus Christ on the cross,” Everhart said as she began the second day of the state GOP convention. “He never denounced God, either.”
Everhart was praising McCain for never denouncing the United States while he was being tortured as a prisoner of war in Vietnam.
Examples of the same with respect to McCain’s presumed opponent in November are legion and therefore redundant. But while I’m at it, here are some more political nimbus photos: