Check out this entertaining trope from William Kristol’s op-ed today in the New York Times.  He gives three arguments that have the form of the enthymeme–the argument missing a conclusion or premise–but none of the validity necessary to make such arguments effective.   He writes:

But orators often ask themselves the convenient questions, not the difficult ones. And Barack Obama is an accomplished orator.

And

After all, politicians sometimes indulge in ridiculous and unfair
comparisons to make a point. And Barack Obama is an able politician.

And

But ambitious men sometimes do a disservice to the best in their own communities. And Barack Obama is an ambitious man.

As you can see, these have the form of a categorical syllogism–a two-premised deductive argument consisting of categorical statements.  Here’s a more famous example:

• All men are mortal, and Socrates is a man.

Same form, save for the first premise.  For Kristol’s arguments here to be valid deductive ones (and perhaps sound ones as well), he needs to drop the qualifier (the "often" or "sometimes").  For his arguments might as well say:

• Sometimes men are mortal, and indeed Socrates is a man.

Is Socrates mortal?  Can’t really tell by that argument.  Does Barack Obama do the things in the arguments above?  Can’t really tell.  He might or he might not.  But it doesn’t follow from the fact that he is a politician that he will engage in "ridiculous and unfair comparisons to make a point." 

Well, ok.  If we stretch the principle of charity to the breaking point, we might read the first premise another way:

• All orators often ask themselves convenient questions. . . .
• All politicians sometimes indulge. . . .
• All ambitious men sometimes. . . .

That would make Kristol’s enthymemes valid, but ridiculously unsound.  Besides, that’s not what he means.