Digging up dirt on the opponent has become standard operating procedure for major political campaigns in America.

In fact, it’s an entire discipline within the operation of political campaigns. It has its own name — opposition research, or “oppo.”

Some campaign workers spend their entire careers doing nothing but opposition research, compiling books of the stuff on political opponents. Joe Sinsheimer, the fellow who formed the jimblackmustgo.com Web site and doggedly pursued the now-imprisoned former House speaker, was an opposition researcher. So too was the late Lee Atwater, the notorious campaign aide for George H.W. Bush.

And nearly 25 years ago, the Republican National Committee created The Opposition Research Group as a separate division within the party.

The information gleaned from these efforts becomes the basis for the negative, mudslinging ads that we see leading up to elections.

Much of the stuff is never used because it is based on rumors and speculation. But whatever skeletons the major-office candidates stick in whatever closets or trunks, the opposition is often well aware of them.

When the campaign for the Democratic nomination for governor began last summer, political insiders speculated that Richard Moore would use two episodes in the life of rival Beverly Perdue against her — an oddly-resolved car accident and an illegal contribution from a rest home mogul. He did.

Both the Perdue and Moore campaigns made public records requests of each other’s offices to further their opposition research.

But these days, Perdue’s Republican rival for governor is crying foul over a similar request.

The Democratic Party has requested correspondence between Charlotte Mayor Pat McCrory, the city manager, and three companies with a huge presence in the city — Duke Energy, Bank of America and U.S. Airways. Party officials want information going back 13 years.

McCrory calls the request “dirty tricks.” He says it’s a fishing expedition being conducted “with the blessing of Beverly Perdue.” He’s certainly right about the fishing expedition. And he’s probably right about Perdue’s blessing.

But dirty tricks? Well, yes, it is a bit dirty, just like all major political campaigns these days. If McCrory didn’t want these fleas, he shouldn’t have started running with these dogs.

His complaint does raise an interesting question, one that should be put to him: Does this mean that his campaign won’t be making similar requests of the lieutenant governor’s office? Does it mean that his campaign won’t be conducting opposition research that involves public-record requests of any kind?

The Charlotte mayor makes a valid point that the cost of meeting the request could cost Charlotte taxpayers tens of thousands of dollars. The public-records law allows the cost of copying documents to be passed onto the person requesting them. Perhaps a review is needed to see whether that cost is sufficient in all cases.

But like or not, this is politics in America today. Some would argue that it’s always been this way and that only the technology and the means have changed.