Former Florida governor Jeb Bush speaks during a town hall meeting with Puerto Rico's Republican Party in Bayamon, Puerto Rico. (Ricardo Arduengo/AP)

Fox News announced guidelines Wednesday that will winnow the field of participants in the first Republican debate of the 2016 presidential campaign.

The network will require contenders to place in the top 10 in an average of the five most recent national polls in the run-up to the event, narrowing what is expected to be a field of 16 or more by the Aug. 6 event in Cleveland.

The rule could trigger an early rush of spending by lower-tier candidates seeking to boost their standing in national surveys before the pivotal first forum.

Meanwhile, CNN laid out a different approach for the second debate on Sept. 16, which will be split into two parts — one featuring the top 10 candidates in public polling and a second that will include lower-tiered candidates who garner at least 1 percent in polls. The forum, being held at the Ronald Reagan Presidential Library in Simi Valley, Calif., will also require participants to have at least one paid campaign worker in two of the four early voting states.

Determining which contenders will get to participate in the official forums sanctioned by the Republican National Committee has been a challenge for the cable news network and party officials. No GOP primary debate has ever featured more than 10 candidates.

RNC Chairman Reince Priebus said Wednesday that the party supports the approaches taken by the two cable networks.

It remains to be seen how many candidates will be included in the Fox News debate under the criteria, which could allow more than 10 participants if some are tied in the polls.

The top 10 contenders in the five most recent national polls are former Florida governor Jeb Bush, Sen. Marco Rubio of Florida, Wisconsin Gov. Scott Walker, Sen. Rand Paul of Kentucky, Sen. Ted Cruz of Texas, former Arkansas governor Mike Huckabee, retired neurosurgeon Ben Carson, New Jersey Gov. Chris Christie, real estate tycoon Donald Trump and former Texas governor Rick Perry, according to a Washington Post analysis. Former U.S. senator Rick Santorum of Pennsylvania and Ohio Gov. John Kasich are a fraction of a point behind Perry.

Lagging behind those 12 are Sen. Lindsey O. Graham of South Carolina, Louisiana Gov. Bobby Jindal, former Hewlett-Packard chief executive Carly Fiorina and former New York governor George Pataki.

Without Fiorina and Jindal, the opening event of the GOP primary contest would feature an array solely of men, most of them white — not the image that the party wants to present as it seeks the support of an increasingly diverse electorate. Representatives for Fiorina and Jindal declined to comment.

Fox News plans to provide additional coverage and air time Aug. 6 to the candidates who do not place in the top 10, according to the details, which were first obtained by The Post.The criteria are similar to the standards Fox News has set for past debates. To qualify, candidates must place in the top 10 of an average of the five most recent national polls by Aug. 4 at 5 p.m. Eastern time. Such polling must be conducted by major, nationally recognized organizations that use standard methodological techniques.

Debate participants must also meet all U.S. constitutional requirements to run for president, must have announced their campaign and filed the necessary paperwork with the Federal Election Commission, and must have paid all required federal and state filing fees.

The debate, which Fox News is presenting with Facebook, will be moderated by Bret Baier, Megyn Kelly and Chris Wallace.

Scott Clement contributed to this report.