The biggest problem with President-elect Barack Obama choosing Hillary Rodham Clinton as his Secretary of State is obvious: With the Clintons, you always get two for the price of one.

Would No Drama Obama be able to deal with Bill's perpetual burlesque? How do you tell a former president not known for humility or discretion to check himself? Why is he speculating in public about negotiations on his wife's behalf? Mr. Clinton could turn out to be the most troublesome Cabinet spouse since Martha Mitchell let her fingers do the walking.

Second, there's Mrs. Clinton's suitability. Nobody doubts that she's smart and hard working, and certainly she would bring to the job of America's top diplomat a wealth of international goodwill left over from her husband's administration. But does she have the diplomatic skills to do what's required of her? Does she have the managerial aplomb to run the State Department and U.S. foreign policy? That she couldn't settle down a dysfunctional campaign staff does not augur well.

But the most substantive objection is personal: Could President Obama trust her? And, relatedly, would foreign leaders know there was no distance between her and the president?

She has been, of course, Mr. Obama's chief Democratic rival. Some say that's not a problem; Abraham Lincoln made a point of assembling a "team of rivals" in his presidential Cabinet. Which sounds fine until we recall that the Lincoln strategy was not a successful one. The Democratic primary season left ample reason to wonder whether a Secretary of State Clinton would look out first for the president's interests — or her own.

When a secretary of state is close to a president — think James Baker and George H.W. Bush — the machine of U.S. diplomacy runs smoothly and effectively. But when he or she isn't — Colin Powell and George W. Bush come to mind — it sputters.

Nobody doubts Mrs. Clinton's fidelity to the Democratic agenda. But it's hard to believe there's enough room in one Cabinet for two exceedingly ambitious Democrats — or, frankly, three.