Karl Rove used to say that to take down an opponent, you don't go after his weakness, you go after his strength. If you can take away whatever gives him his greatest appeal, he'll be left with nothing.

In their desperate attempts to weaken Donald Trump and deny him their party's nomination, Republicans have finally gotten around to following this advice. After saying Trump isn't a real conservative, saying he doesn't understand policy, and saying he's an erratic bully with ugly views — all of which are true, and all of which did nothing to slow his rise — they are at last trying to get voters to take a good hard look at Trump's business career.

It's not that they aren't still making those other arguments, because they are, even if Trump's voters don't seem to care. But Marco Rubio is now calling Trump a "con artist," and in his speech on Thursday, Mitt Romney made this case about Trump:

But wait, you say, isn't he a huge business success that knows what he's talking about? No he isn't. His bankruptcies have crushed small businesses and the men and women who worked for them. He inherited his business, he didn't create it. And what ever happened to Trump Airlines? How about Trump University? And then there's Trump Magazine and Trump Vodka and Trump Steaks, and Trump Mortgage? A business genius he is not.

Mitt's right — all those businesses are defunct. The key to understanding them is that Trump is a particular kind of businessman, perhaps a unique one. While he still calls himself a developer, the truth is that over the years his business has been less and less in building things, and more and more in licensing his name. For instance, 17 buildings in Manhattan say "Trump" on them, but he owns only a few. He has figured out a lucrative and generally low-risk strategy: license the Trump name to other people who are building things — a golf club in Puerto Rico, or a condo in Uruguay — and get paid for doing nothing. If the enterprise goes kaput, Trump has no investment to lose.

Which is a clever strategy, without doubt. It's built on the principle that what matters is not the product but the brand — or rather, that the brand is the product.

Trump has known this since he was young, which is part of the reason he made himself a celebrity back in the 1980s. It's why he puts his name in big letters on buildings, and why The Apprentice was so key to maintaining the brand and setting him up for a presidential run. Every week, viewers would see people grovel before him, get a tour of some property supposedly dripping with opulence, and see him decisively determine his supplicants' fates. (For a peek, you can watch this segment from one episode touring Trump's apartment, which is a truly spectacular monument to garish bad taste.)

If Republicans — and Democrats, should Trump wind up being the GOP nominee — are looking for an Achilles heel, this is where they'll probably find it. His brand is built on equating the name "Trump" with wealth, success, wealth, luxury, and oh yeah, wealth. It's why he gets unusually sensitive when people say that he might not be quite as rich as he says he is (when he was roasted by Comedy Central a few years ago, the participating comedians were told they were not allowed to make any jokes suggesting that his fortune isn't as big as he says).

Of course, whenever anyone attacks him on these grounds, Trump will respond by basically saying, "Hey, everybody knows I'm rich and successful, so shut up." Everybody knows that now, but everybody might be persuaded otherwise, or at least be persuaded that his wealth comes from scamming people out of their money. In the financial disclosure he released (which many analysts thought was grossly exaggerated), he valued the Trump name at $3.3 billion. Maybe that's true or maybe it isn't, but the name only has the value others agree to ascribe to it.

Many of Trump's supporters have been convinced that if Trump is so rich, he must really know what he's doing, and maybe he can get our government into shape. But if you can convince people — voters or customers — that "Trump" actually denotes a crappy product like Trump Steaks or Trump University that claims to be something it isn't, then nobody has any reason to buy anything with that name on it. And that may be the way to beat him.