Who Could Be the Independent Candidate Against Donald Trump?

Republican leaders are calling for another candidate, but it may be too late.

But the speculation hasn't ended now that Trump is the presumptive Republican nominee; it's now focused around people who could run against him.

Prominent Republican Bill Kristol, who is the editor and founder of the conservative magazine The Weekly Standard has been actively predicting another person will run.

In an email to ABC News, Kristol posed a question rather than giving specific descriptions of his ideal candidate.

"Key question: can a (relative) unknown who's a very impressive person (and will have access to resources and a strong team) run as a citizen candidate who's served his country and establish a contrast to Trump and Clinton?" Kristol wrote.

ABC News political analyst Matthew Dowd said the state of the two presumptive nominees has likely led to Kristol's comments.

"The reason this keeps gets talked about," Dowd said on "Good Morning America," "is that we have two very unpopular likely nominees of the major parties. It creates a window of opportunity."

Kristol has been one of the most vocal advocates for an independent run and Dowd likened his comments to those of the police commissioner in Batman comics who called on the caped crusader to save the day.

For his part, Trump has dismissed Kristol's claims, saying the editor has "no credibility" since Kristol initially said that Trump would never run for office.

"He looks like such a fool," Trump said.

If a third-party candidate were to step up, it would have to be soon since time is running out.

Philip Wallach, a senior fellow at The Brookings Institution, thinks that the later it gets in the race, the bigger a network the individual would need to make their bid viable.

"At this point, it is late enough in the game that only someone with extensive backing and a strong national reputation could have a significant impact as an independent," Wallach told ABC News.

"Because of this, the Libertarian ticket headed by two well-liked former Republican governors has the best chance in its history to tap into the 'None of the Above' vote and expand their party’s backing beyond a highly committed fringe," he said.