Donald Trump may have to decide on that third-party run as soon as Sept. 30

If Donald Trump wants to participate in the South Carolina Republican primary, he'll need to have ruled out any plans for a third-party bid by the end of September, CNN reports.

Trump has repeatedly threatened the Republican Party with an independent bid, even though he has a commanding lead in the polls, with the latest from South Carolina showing him garnering the support of 30 percent of respondents. But he will need to submit a signed pledge to the GOP by Sept. 30 at the latest if he wants to compete in the Palmetto State's primary.

Marco Rubio, Jeb Bush, John Kasich, and Ben Carson have already filed their paperwork, and thereby affirmed that they "generally believe in and intend to support the nominees and platform of the Republican Party in the Nov. 8, 2016, general election." And if Trump were to sign the form and renege on his promise, he might face legal action, according to Matt Moore, the chairman for the South Carolina GOP.

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"It has not been an issue with national candidates in the past," Moore told CNN. "We're hoping it's not this time around."