Jeb Bush (David Paul Morris/Bloomberg)

Mitt Romney, it is widely said, lost the GOP pre-primary race for the establishment vote and big money donors. But on this I agree with Nate Cohn who argues, “The fact that Mr. Romney failed to attract much support in the invisible primary is partly a reflection of his own weaknesses. But it is also a reflection of the relative strength of the field, and perhaps especially Jeb Bush, the former Florida governor who launched an aggressive, even pre-emptive campaign to recruit support in early December.” He contends, “It was not a given that Mr. Bush would receive a warm enough reception from the G.O.P. elite to dissuade Mr. Romney. Yes, Mr. Bush is a scion of the establishment and the potential inheritor of a vast network tied to his brother and father. But he also hadn’t been elected since 2002; his surname is potentially a disadvantage in the general election; and his message — focused on issues like immigration and education — has been somewhat out of touch with the mainstream of his party.”

It is not hard to figure out how Jeb Bush did this if you speak to past Romney supporters and operatives who switched to Bush this time out. Former Romney donors will go to three or four different candidates, but according to GOP insiders I have spoken with since Romney left the field, the overwhelming majority of the bundlers are very hungry for a win and think Bush is smart, focused and vetted. But what about Bush fatigue? Richard Grenell, who worked briefly on the Romney team and continued to support him in 2012 e-mails, “The fear from others of another Bush on the ticket dissipates after hearing Jeb speak and realizing another Clinton is around the corner.  I’ve seen people do a 180 after attending a Jeb Bush event.” He continues, “The more people hear, the better they feel about Jeb.  The long primary process is going to help him.” In other words, Bush has the presidential gravitas that impresses these people and the organization they think is needed to go for months and months. The smart betting is that Bush will wind up with a substantial majority of the Romney donors.

Sources in the donor community say that large numbers of freed up former Romney donors, especially outside the Northeast, are steering away from the camp of New Jersey Gov. Chris Christie. (Christie took another hit when, not long after an earnest appeal in the Freedom Summit, he clocked in at 6 percent in the Des Moines Register/Bloomberg’s Iowa poll — with a 54 percent unfavorable rating.)

If this is accurate, Bush does become the odds-on favorite for the “establishment” candidate, but that does not make him necessarily the favorite to win the nomination. With Romney stepping aside and Bush’s money operation in high gear, Bush now needs to attend to his message, retail politicking and self-definition. He cannot let his opponents define him, nor let voters and the media come to accept a false narrative that he is a squishy moderate. Neither can he engage marginal candidates like Sen. Rand Paul (R-Ky.) who can only grab a news cycle by attacking him with personal slurs and sophomoric stunts. (Commenting on Paul’s fake phone call idea, The Post noted, “The question for Paul, is why? Why bother? Why the snark? Especially over having a name that’s famous in presidential politics? Someone at Paul’s political action committee thought this was a good idea. Perhaps it would go viral. Perhaps the youngsters on the Twitter would love it. Maybe it would make Paul seem hip. But the pursuit of cool never ends well.”)

While Bush is working on all that, the  challenge for each Bush rival is to become the most credible, best prepared and most electable alternative to Bush. Bush can outlast and wear down a big field of 10 or more opponents, but one or two not-Bush candidates who rise above the fray may be able to capitalize on concerns about Bush and reassure the party he is a better nominee to go up against the Clintons.

First, Bush is not Romney but neither is he an everyman. No one outside his siblings can claim to be the sibling of a president and the son of another. He is not clueless like Clinton and he is not prone to claims of poverty. Still, he cannot tell the sort of shopping-at-Kohl’s story that ingratiates Wisconsin Gov. Scott Walker with voters or explain how little his parents had when they came to America as immigrants the way Sen. Marco Rubio (Fla.) can. Telling a unique story of personal struggle and relating on a very human level with the stresses of average Americans can lift a contender like Walker above a field of rhetorical firebrands and novice politicians.

Second, the not-Bush leader will be someone who can fill competing requirements; He must be fresher than Clinton but possess enough gravitas not to be intimidated by her or look like a novice in comparison. A senator with little or no record will find that hard. (Rubio, by the way, has 14 years as a state speaker and would do well to sketch out his accomplishments there.) As former Texas governor Rick Perry put it, “Would you rather have someone who talks about aviation a lot and really gives a great speech about aerodynamics and about navigation and about weather and about all of the different things a pilot needs to know, but have only about 100 hours of being behind the yoke of an airliner? Or would you rather have a 10,000-hour individual who has had to fly through storms?” (That may explain why Texas freshman Sen. Ted Cruz languishes in single digits in early polling.)

It is a Goldilocks challenge — someone experienced but not stale, someone young but not immature, someone who generates GOP excitement but does not scare independents. The not-Bush will need to be seen as able to take better advantage of Clinton’s staleness, dullness and sense of entitlement.

Third, the not-Bush will need to be well versed in foreign policy, not only to be a credible contender against Clinton but also to take on Bush. Bush is well-versed in national security and well-traveled, comfortable on the world stage and presents a presidential persona. The need for a 21st century Republican foreign policy that rejects the passivity of the Obama years, restores military power and presents a coherent strategy for defeating jihadist terrorists has never been greater. The successful not-Bush will need just such a vision. The not-Bush is running for commander in chief, not secretary of state but he nevertheless must be firmly in command of his facts and show an appreciation for the complexity of our international challenges.

Lastly, the not-Bush will need to be the party healer, not the representative of the non-establishment wing of the party. The entire appeal of a not-Bush candidate would be his ability both to unite the party and to extend its reach. Cruz is not a uniter, nor is a Mike Huckabee. Both represent and play to factions of the party. A tough, happy Republican who is feisty enough for the grassroots and solid enough for the insiders can fill that role. The party needs someone who does not drive any significant segment of the party to distraction.

In sum, the next few months may tell us how quickly Bush can occupy the establishment side of the ledger and which candidate(s) can match him stride for stride for 12 months. Rubio and most especially Walker (who shot to the top of the pack in early Iowa polling) as well as the longest serving GOP governor with the best job-creating record (Rick Perry) are capable of laying claim to the top non-Bush spot. It’s the contest among the not-Bush contenders that may now be the most interesting and critical part of the GOP primary process.