WASHINGTON - Former First Lady Nancy Reagan is inviting 16 of the all 17 Republican presidential candidates to ascend the debating stage in the second prime-time TV debate hosted by the US TV network CNN and Reagan Library on Sept 16, CNN said Tuesday.

The network did not explain why former Virginia Governor Jim Gilmore has not yet been invited so far. However, it said candidates must achieve an average of at least 1 percent of support in three recognized national polls before Sept 10 to get the tickets to the CNN/Reagan Library debate.

According to CNN, the second Republican primary debate will be broken into two back-to-back debates with two groups of candidates, a format different from that of the first Fox primary debate, where two debates hosted separately the top 10 Republican candidates in approval ratings and the rest seven.

The TV network did not elaborate on how it would divide the candidates into two groups.

Meanwhile, CNN said its anchor of the news program "State of the Union" Jake Tapper will moderate both groups participating in the debate.