When Donald Trump takes the stage on "Saturday Night Live" this weekend, federal regulators may be watching closely, too.

A little-known TV rule at the Federal Communications Commission could be used to guarantee "equal time" on TV for Trump's rivals in the Republican presidential primary, due to a clause that aims to ensure "equal opportunities" for qualified candidates seeking the same office.

Although candidates can't take advantage of the equal time regulation when their competitors are mentioned on news segments or appear in news interviews, they can use it when it comes to other broadcast shows, which SNL happens to be.

Politicians haven't made a habit of invoking the rule, but the issue has come up in this campaign season. After Democratic candidate Hillary Clinton appeared on SNL last month, NBC — which broadcasts the show — sent a memo to its network affiliates warning them that they could be asked to provide equal time.

Clinton appeared on NBC for a total of three minutes and 12 seconds, the memo noted, a fact that longshot hopeful Larry Lessig pointed out in asking for equal time just a couple weeks later. Lessig has since dropped out of the race.

When asked how the FCC viewed the issue in light of the current campaign, chairman Tom Wheeler told reporters that the rules were clear.

"Rules are rules," said Wheeler. "I hope that we have developed a reputation as folks who enforce the rules."

Why are candidates so eager to host a show like "SNL" that will undoubtedly make fun of them for an hour?