One obvious difference is the size of the Republican field -- 17, before Walker's withdrawal and Rick Perry's withdrawal 10 days before. That has made debates unwieldy and has placed a premium on somehow emerging from the scrum.
Walker didn't. He spoke for just seven minutes in the two-hour Fox News debate on Aug. 6 and the three-hour CNN debate (in which he was asked just three questions) on Sept. 16. It was not a format that favored Midwestern reticence.
Walker's withdrawal was unexpected also because, after the rise of billionaire-funded super PACs kept Newt Gingrich and Rick Santorum plugging on in the 2012 cycle, it seemed a candidate with such backing had no reason to quit. But super PACs can't pay for candidates' staffs, offices and travel, and after the debates Walker couldn't raise enough to pay for them.
In previous cycles, it was assumed that voters were looking for candidates who could identify with ordinary people and the struggles they face, and Walker seemed typecast for that, saving up coupons to shop at Kohl's. But today's poll leader is a Manhattan billionaire who boats of (and perhaps inflates) his wealth and glories in his vulgarity.
It has long been taken as a given that a successful candidate must concentrate on winning one of the early contests -- the Iowa caucuses, the New Hampshire or South Carolina primary (no one has quite figured out how to target Nevada). Rudy Giuliani's failure to settle on one state and demonstrate what he had in common with Republicans there led to his quick withdrawal in 2008.
Walker's obvious target was Iowa, next door to Wisconsin, demographically similar, where the candidate was born and spent his early years. His speech at the January Freedom Summit in Des Moines wowed the audience and rushed him to the top of polls in Iowa -- and New Hampshire and nationally as well.
That was a sign that, with the advent of new media, the contest had become nationalized. Walker nevertheless zeroed in on Iowa. He stressed his strong religious beliefs in a state whose caucus has had the largest concentration of evangelical Protestants of any contest outside the South.
Having accepted blandly a court's legalization of same-sex marriage in Wisconsin, he reacted to the Supreme Court's similar decision by endorsing a constitutional amendment reversing it -- a proposal that was going nowhere.
He continued to stress his steadfastness in the face of furious opposition and threats of violence from public employee unions in Wisconsin, but he failed to do enough homework and settle on consistent responses to questions on national and foreign policy.
The result was stories about his unwillingness to answer questions on a trip to London and directly contradictory statements on whether the Fourteenth Amendment confers birthright citizenship.
He lost his lead in national polls by April, as other candidates got announcement-day bounces, and in New Hampshire polls by late May, two weeks before Trump's announcement. In Iowa he was overtaken by Trump in the first week in August, before the first debate.
Trump's rise (and, more recently, the surges for Ben Carson and Carly Fiorina) has been ascribed to Republican voters' desire for a candidate not besmirched by government and Washington. But I think what may be happening is that the Republican electorate is expanding to include many outside the familiar categories of economic, religious and libertarian conservatives.
Walker, with his fight against public employee unions and frequent invocations of faith, with his small town upbringing and suburban political base, had the potential of appealing to the familiar groups. But not, it seems, to those seeking a champion from outside the political world.
Certainly the Republican electorate has been open to expansion. In Iowa, Walker's chosen target, only about 100,000 people have been attending Republican caucuses in a state of 3 million. Democrats, who run about even in statewide races there, have been attracting more than 200,000.
The huge audiences for the Republican debates -- 23 to 24 million, nearly triple previous highs -- also suggest that many more Americans are interested in the party's contest this cycle.
While withdrawing, Walker urged others to coalesce around someone who could beat the (unnamed) poll leader. That hasn't happened in other cycles. But this time seems different.