For all of Mitt Romney’s talk of what he would do on Day One in the White House — Bomb Iran? Or was it Planned Parenthood? — there’s just as good a chance he would be tacking up two pictures on the wall. One would be of George H.W. Bush and the other of Jimmy Carter. They both became one-term presidents after they were challenged in the primaries. This is a lesson for Romney.

It is also a lesson for everyone who thinks that if Romney becomes president, he would govern from the center. This is a widely held belief, encouraged by the Romney camp itself and the supposed gaffe of Eric Fehrnstrom that the world would see a different Romney in the general election: “Everything changes. It’s almost like an Etch a Sketch. You can kind of shake it up and we start all over again.” This is not a gaffe but a feint. Romney would be able to restart nothing.

In the first place, Romney would likely have a Republican House, and maybe a Senate, too. This means he has to work with a party that has just recently punished Richard Lugar for excessive moderation and is willing, at this very moment, to bring down the country’s credit rating another notch rather than budge on the debt ceiling. To Romney, who made a fortune with the clever prestidigitation of debt, this has to make no sense, but he would go along because (1) he’d have to, and (2) he always does.

Congress, though, would be the least of President Romney’s troubles. The real threat will come from the Republican Party’s very core, which likes him little and trusts him less. The moment he shows the slightest moderate or rational tick, someone such as Rick Santorum will barrel out of the GOP’s piney woods, screaming oaths, and enter the 2016 Iowa caucuses that, you might remember, Santorum won in 2012. He must be itching for such a fight, having already called Romney “the worst Republican in the country to put up against Barack Obama.” That, folks, is not a fudge.

As luck would have it, the Supreme Court has enabled any billionaire to effectively fund a presidential campaign. Santorum’s guy was Foster Friess, who anted up $2.1 million for the Red, White and Blue Fund, but there are plenty of others. It took a herculean fundraising effort by Pat Buchanan to challenge the elder Bush in 1992 (and get 37 percent of the vote in New Hampshire), but it now takes one guy. The conservative movement is lousy with such people, rich men who play with politics as they once did with electric trains.

It’s hardly conceivable that, as president, Romney will become the Romney some think he is. The forces that shaped him in the primaries and caucuses will not go away. He has been clay in the hands of the political right, and this will not change. After Romney recently disparaged Carter’s political courage, Gerald Rafshoon, once Carter’s communications director, shot back with this via Bloomberg View: “Scour Romney’s record for a single example of real political courage — a single, solitary instance, however small, where Romney placed principle or substance above his own short-term political interests. Let me know if you find one.” Rafshoon’s phone has not been ringing.

The widespread belief that Romney would govern from the center is supposedly supported by the equally widespread belief that he is a liar. I hear this all the time: Never mind what Romney said in the primaries, he is a moderate Republican. These people point to Romney’s record as the moderate governor of liberal Massachusetts — even though he has renounced his moderation, as if it was an unaccountable episode of mental instability. The belief that he would revert is the desperate rationale of nominal Democrats who have had it with Barack Obama and want to be excused for abandoning ship. (In the business community, little distinction is made between Obama and Leon Trotsky, another community organizer . . . so to speak.)

According to what a family friend told the New York Times, Mitt and Ann Romney decided he should run for president because they both “felt it was what God wanted them to do.” Having done just that, Romney has left it to others to define what sort of candidate he would be. Nothing would change if he were president. Weakness is his one consistency.