(Adds CNN debate plans, paragraphs 4-5)
By Peter Cooney
WASHINGTON, May 20 (Reuters) - The first debate among Republicans seeking their party's 2016 presidential nomination is scheduled for Aug. 6 in Cleveland, but not all the hopefuls will get a chance to bask in the national television spotlight.
Fox News, which will host the debate with Facebook, said on Wednesday that contenders seeking to participate must place in the top 10 of an average of the five latest national polls as recognized by the network leading up to Aug. 4.
That is likely to mean no room for a few hopefuls on what will already be a crowded stage. Six Republicans have already declared their candidacies and they could be joined by at least eight more by summer.
Later on Wednesday, CNN said it would host a Republican debate on Sept. 16 at the Ronald Reagan Presidential Library in California. It said the event would involve two parts - one involving the top 10 contenders in opinion polls and the other for those outside that group but meeting a minimum polling threshold of 1 percent.
The Republican National Committee said it supported and respected both cable news networks' decisions.
Republican Party leaders have been struggling to figure out who among their likely crowd of 2016 candidates deserve to take the stage in debates.
Many party officials believe the maximum number that can fit on the debate stage and fully participate is 10 or 12.
Their analyses from debates in previous campaigns show that in a 90-minute debate with 10 people on stage, each candidate gets only about 4-1/2 minutes of speaking time.
But officials worry that limiting the debate field could ignite a backlash among those left out.
In a realclearpolitics.com average of Republican presidential polls so far, the top 10 contenders were former Florida Governor Jeb Bush, Wisconsin Governor Scott Walker, Florida Senator Marco Rubio, Kentucky Senator Rand Paul, former Arkansas Governor Mike Huckabee, Texas Senator Ted Cruz, retired physician Ben Carson, New Jersey Governor Chris Christie, former Texas Governor Rick Perry and former Pennsylvania Senator Rick Santorum.
Those on the outside looking in were Ohio Governor John Kasich, former Hewlett-Packard Chief Executive Officer Carly Fiorina, Louisiana Governor Bobby Jindal and South Carolina Senator Lindsey Graham.
Fox News added it would "provide additional coverage and air time on August 6th to those candidates who do not place in the top 10."
(Reporting by Peter Cooney; Additional reporting by Steve Holland; Editing by Eric Beech)