By Tim Reid

LOS ANGELES Donald Trump's rivals need to take him on more forcefully at next week's Republican presidential debate if they have any hope of cutting into the runaway front-runner's massive lead in opinion polls, one of the debate moderators said on Tuesday.

"They're not going to be able to put a dent in his lead if they don't. President Trump has a commanding and seemingly very enduring lead. It's up to these candidates to show why they think that they would be better," said Dana Perino, a White House press secretary under Republican President George W. Bush and now an anchor and political commentator at Fox News.

Trump, who leads his nearest rival for the Republican presidential nomination by some 40 percentage points in opinion polls, is skipping the second debate, just as he did the first one in Wisconsin last month.

Perino, a co-moderator of the Sept. 27 debate, said none of Trump's rivals managed to alter the dynamic of the nominating race in that first debate. She said that needed to change in the next rhetorical clash, which will take place at 9 p.m. ET (0100 GMT on Sept. 28) at the Ronald Reagan Presidential Foundation and Institute in Simi Valley, California.

"If you're a supporter or a donor of one of these candidates, you want to see a breakout moment," she said.

With voting in the nominating contest starting in Iowa in January, Trump's Republican rivals are running out of time to halt his march to becoming the Republican standard-bearer, despite his myriad legal troubles. Trump has been indicted four times this year.

Stuart Varney, a Fox Business Network anchor and one of Perino's co-moderators, said the candidates will be questioned on a range of issues, including immigration, inflation, crime, and foreign policy.

Six candidates have so far qualified for the debate: Florida Governor Ron DeSantis, former Vice President Mike Pence, former U.S. ambassador to the U.N. and former South Carolina Governor Nikki Haley, U.S. Senator Tim Scott of South Carolina, biotech investor Vivek Ramaswamy, and former New Jersey Governor Chris Christie.

Most of them are languishing in single digits in opinion polls, way behind Trump, whose lead has only increased in recent months with each of four criminal indictments.

The Republican National Committee, which organizes the debates, has picked the Fox Business Network to host the event, alongside Univision, the U.S.-based Spanish-language TV channel, and Rumble, an online video platform popular with conservatives.

(Reporting by Tim Reid in Los Angeles; editing by Ross Colvin; editing by Jonathan Oatis)