As he tries to engineer a comeback in this week’s presidential debate, President Obama needs to recognize two things. First, when it comes to politics, Mitt Romney treats himself as a product, not a person. Second, Republicans cannot defend their proposals in terms that are acceptable to a majority of voters.

You can imagine Romney someday saying: “Politicians are products, my friend.” There’s no other way to explain why a candidate would seem to believe he can alter what he stands for at will. His campaign has been an exercise in identifying which piece of the electorate he needs at any given moment and adjusting his views, sometimes radically, to suit this requirement.

In that respect, Romney does Richard Nixon one better. When Nixon was looking to revive his career in the 1968 campaign, the terribly scarred veteran of so many political wars realized his old persona wouldn’t sell. And so he created what came to be known as the “New Nixon” — thoughtful, statesmanlike and tempered. The operation worked until Nixon’s old self got him into trouble.

But manufacturing the New Nixon took years of painstaking effort. New Romneys appear on a monthly, weekly and sometimes daily basis. Thus did Romney move far to the right on immigration last year because he needed to dispatch nomination rival Rick Perry, a moderate on that one issue. Since then, Romney has been trying to backtrack to appease Latino voters.

During the same nomination battle, Romney abruptly changed his tax policy to placate the supply-side-Wall-Street-Journal-Grover-Norquist axis in the GOP. Romney’s initial tax proposal was relatively modest. The right wasn’t happy. No problem, said Romney, and out came his new tax plan that included a 20 percent cut in income tax rates, “rate cuts” being a term of near-religious significance to supply-siders.

Romney pointedly asserted (again, in the primaries) that he wanted the tax cut to go to everyone, “including the top 1 percent.” But this doesn’t sell to swing voters now, especially after the leaked video in which Romney wrote off 47 percent of Americans as incorrigibly dependent. So in the first debate, Romney tried to pretend that he didn’t want to cut rich people’s taxes. He reassured us that “I’m not going to reduce the share of taxes paid by high-income people.” (By the way, he could cut taxes for the rich a lot and still keep their “share” of the government’s overall tax take the same.)

And then there’s abortion, an issue about which you have to wonder if Romney cares at all. Without much effort, you can find video online in which Romney declares with passion and conviction that he is absolutely committed to a woman’s right to choose — and video in which he declares with equal passion and conviction that he is absolutely opposed to abortion and committed to the right to life. Just recently, Romney moved again, offering this shameless gem of obfuscation to The Des Moines Register editorial board: “There’s no legislation with regards to abortion that I’m familiar with that would become part of my agenda.” There is no candidate I am familiar with who has tried to have as many positions on abortion in one lifetime as Mitt Romney.

But there’s an underlying reason for Romney’s shape-shifting. It’s the same reason Rep. Paul Ryan always resorts to impressive-sounding budget speak and mathematical gobbledygook to evade explaining the impact of his budgets on actual human beings.

Romney, Ryan and the entire right know that their most deeply held belief – the one on which they won’t compromise — is rejected by the vast majority of Americans. That’s their faith that every problem in the economy and in society can be solved by throwing more money at rich people through tax cuts.

Vice President Joe Biden kept Ryan on the defensive during most of Thursday night’s debate precisely because he refused to let anything distract him from driving this central point home. Without pause and without mercy, Biden kept bringing viewers back to the obsession of the current Republican Party with “taking care of only the very wealthy.”

Obama doesn’t have to look angry or agitated in this week’s debate. He simply needs to invite voters to see that Romney, the product, will give them no clue as to what Romney, the person, might do as president. Romney keeps changing the packaging because he knows that the policies inside the box are not what voters are looking for.

E.J. Dionne’s e-mail address is ejdionne(at)washpost.com.