Though Donald Trump is maintaining a healthy lead in polls for the Republican presidential nomination, there is mounting evidence that he has already peaked.

It's true that Trump has defied many predictions up to this point in his candidacy. Bill Kristol, editor of the Weekly Standard (which has the same ownership as the Washington Examiner), has for weeks been hypothesizing that Trump had peaked, only to see him remain at the top of the crowded GOP field. But the data suggest that Kristol may be on to something.

Though there hasn't been any indication of a precipitous drop in Trump's numbers, his support seems to have met resistance at around the 25 percent mark after weeks of rising. Though that's still good enough to give him a double-digit lead in a crowded Republican field, it won't be as the field narrows.

Past Republican candidates, such as Ron Paul in 2012 and Pat Buchanan in 1996, had a core base of dedicated followers, but had too low of a ceiling to capture the nomination. Trump seems to be in a similar position.

On June 16th, when Trump announced he was running for president, he was polling at 3.6 percent, according to an average of national polls compiled by RealClearPolitics. By August 6th, the day of the first Republican presidential debate, Trump had surged to 24.3 percent. But after that, his numbers leveled off, and as of this writing, his average is down to 22 percent.

Breaking the numbers down further, that average is based on three different polls. One by Rasmussen recorded a significant drop in Trump's support to 17 percent — from 26 percent in the same poll in late July. Another, by Fox, essentially showed no change — Trump was at 26 percent in early August, then slipped to 25 percent.

It's true that CNN's August poll showed Trump gaining 6 points as compared to July — but even so, Trump was at 24 percent.

An analysis by the Huffington Post's Mark Blumenthal noted that traditionally, 25 percent isn't enough to capture the nomination.

Between 1992 and 2012, according to Blumenthal's analysis, the runner-up candidates for the GOP nomination garnered between 20 percent and 33 percent of the vote in the primaries. Candidates who won the nomination, however, did much better — in the 47 percent to 60 percent range.

Right now, between 75 percent and 80 percent of the Republican electorate is not supporting Trump – and there are 10 candidates bunched up, from Chris Christie at 3.3 percent to Jeb Bush at 10.7 percent. As the field gets smaller, it will allow another candidate to surpass Trump.

And this assumes that Trump remains where he is. If this election follows the pattern of the last two Republican nomination battles, he'll collapse as people begin to follow the race more closely.

On Aug. 20, 2007, Rudy Giuliani led the Republican pack with 28.2 percent of the national vote, and a surging Fred Thompson was closing in with 17 percent. When the dust settled, Giuliani didn't claim a single delegate, and Thompson dropped out of the race even before Giuliani did.

Even later in the last presidential cycle, Rick Perry peaked at nearly 32 percent in the national average as late as Sept. 12, 2011, before collapsing.

Writing at the Daily Beast, savvy Republican pollster Kristen Soltis Anderson offers a word of caution to those predicting Trump's imminent demise. She notes that in this cycle, there is evidence that Republican voters are less concerned about electability in a nominee — either because they've been spurned before by the candidates deemed by party elites to be "electable," or because they view Hillary Clinton as especially easy to beat. She notes recent polling showing Trump within single-digits of Hillary Clinton in a hypothetical head-to-head matchup.

"[W]hat should make Republican leaders nervous isn't that voters are experimenting with a rebellious choice: It might be that they also genuinely think he's electable, too," she writes.

But the CNN poll that received attention for showing Trump ahead of the Republican pack and within 6 points of Clinton, also had warning signs for the Trump campaign.

Of all the Republican candidates, Trump is the most well known among registered voters surveyed — with just 1 percent of respondents saying they never heard of him — and respondents had an overwhelmingly unfavorable view of him, 59 percent to 36 percent. In contrast, 37 percent never heard of Scott Walker, meaning he has room to grow, whereas Trump does not.

When Republican voters were asked whether they had a better or worse chance of winning the presidency with Trump as the nominee, 58 percent said the party would have "a better chance with someone else."

The bottom line: Trump has nowhere to go but down.