If we had been told at the beginning of the year that in October a Republican presidential candidate would be leading the polls nationally and in almost every state, drawing support from across all factions of the GOP and sucking up all the oxygen in the race, most people would have guessed Jeb Bush was going from strength to strength.

Of course, it is his nemesis and tormentor, Donald Trump, who is at the top of the heap. Trump is something new in Republican politics. He’s the anti-establishment front-runner.

Trump lacks many of the attributes of a traditional front-runner — the endorsements, the broad fundraising base (because he’s not really fundraising), the well-oiled campaign apparatus. What he does have is the polling strength, the dominance in the media and the ability to drive the debate. He is not the candidate of any one faction, but performs well across the spectrum and especially with GOP moderates.

He has usurped the position held by the strongest establishment candidate almost by default for decades, and reduced Bush, the presumptive heir to that slot, for now, to a well-funded also-ran.

It’s a different time with different players, but what has transpired so far would be a little like George W. Bush struggling against Steve Forbes, Gary Bauer and Alan Keyes in 2000.

Usually, other candidates measure themselves by the yardstick of the establishment front-runner. How do they match up against him? How can they exploit his vulnerabilities? In this race, it has often seemed that the most important strategic decision a candidate can make is deciding how to react to Trump.

Usually, the establishment front-runner gets disproportionate media attention by virtue of his strong position in the polls and the belief that he really could be president someday. Thanks to his media skills and his ratings, Trump gets this kind of press attention — and more.

Usually, insurgent candidates cluster on the right and cannibalize one another’s support, while a center-right establishment candidate consolidates the middle of the party. This time is different. With the rise of the outsiders, the field has been divided between political neophytes and everyone else — with everyone else fighting for running room the way the insurgents usually do.

It’s not that Trump has the outsider mantle to himself. Ben Carson is running a strong second and probably has more room for growth. It’s just that the outsider market share is so large. If you count Carly Fiorina and Ted Cruz as outsiders (subject to debate), the latest Monmouth poll shows the traditional candidates collectively getting 25 percent.

Trump happened to arrive at a moment of palpable Republican disgust with the Republican Party. This made its voters susceptible to the charms of a candidate who, like Trump, is a walking incendiary device, and resistant to the appeal of a candidate who, like Bush, is almost stereotypically establishment — the big-donor-approved member of a family dynasty who got into the race accompanied by “shock-and-awe” presumptions of inevitability.

Bush became Trump’s perfect foil. And while many of the other candidates have sidestepped Trump, or dismissed him, or (rarely) skewered him effectively, Bush has repeatedly engaged in hand-to-hand combat with him, seemingly oblivious to the fact that it serves Trump’s interests to continually have opportunities to kidney-punch a son of the establishment.

If Trump is thriving for now, many of the qualities that make him different from a traditional front-runner have significant downsides. He’s not a steady hand. He’s not a conservative or even a center-right Republican with a broadly conservative record. He’s never held or run for office. He has little hope of uniting the entire party behind him, and if he stays this strong into next year, he will be the target of a fierce, no-holds-barred counterattack from within the party.

But he’s setting the pace. He’s the anti-establishment front-runner.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.