One cause of sloppy reasoning is fixing the argument around the position rather than the position around the argument.  When you’re settled about what position you must hold, then your options close in around you.  To that end, there is an interesting article on the Monty Hall problem in the New York Times (by John Tierney of all people).  Another cause of sloppy reasoning is simple incoherence.  Richard Cohen, liberal pundit for the Washington Post, is sometimes guilty of this.  Today, for instance, he returns again to the issue of race and Obama.  Here is how he closes his argument:

From time to time, Obama is likened to John F. Kennedy
— both charismatic and inexperienced politicians when they launched
their presidential campaigns. But Obama could be like Kennedy in
another way as well. Kennedy was a Roman Catholic, and no Roman
Catholic had ever been elected president. In the 1960 Wisconsin
primary, he ran into a version of Cohen’s Law. He won the state but did
poorly in Protestant areas. A month later, he won in overwhelmingly (95
percent) Protestant West Virginia and did so because he bought a half-hour of TV time and confronted the religion issue head on. It was a landslide.

Maybe Obama’s Philadelphia speech on race served the same purpose. The results from the upcoming primaries, particularly Pennsylvania,
will tell. My guess is that he still has not put the race issue to rest
— maybe because he failed to do what Kennedy did in West Virginia. In
that speech, Kennedy told Protestant West Virginians that when
presidents took the oath of office, they were swearing to the
separation of church and state. A president who breaks that oath is not
only committing an impeachable offense, he said, "but he is committing
a sin against God." In other words, he told West Virginians that their
major fear was baseless.

Obama in his Philadelphia speech said nothing as dramatic. On the
contrary, when it came to the perceived threat posed by young black men
(one out of every nine is in criminal custody), Obama built a fence
around the issue by citing his grandmother’s "fear of black men who
passed her by on the street" — suggesting it was comparable to what
his former pastor, the Rev. Jeremiah Wright, had said. He did not confront white fears. Instead, he implied that they were illegitimate.

This is not 1988, and much has changed. For one thing, the GOP
nominee is going to be an aging foreign policy hawk with no coattails
to run on. But if the upcoming Pennsylvania primary simply echoes
earlier racial divisions, Obama has to give yet another speech — this
one directed not at the pundits he so enthralls but at the very people
who have so far rejected him on account of race. Will it matter? John
Kennedy proved a long time ago that it might.

In the first place, who are the pundits Obama enthralls?  And why do pundits like Cohen use the word "pundits" as a term of abuse?  He must not consider himself a pundit.  Or maybe he thinks you’re not a pundit if you use the word pundit to describe pundits.  Besides this, he clearly doesn’t read the pundits, for they’re not enthralled with Obama.  They, the pundits that is, often claim that we’re supposed to dislike Obama on account of his popularity among people, not pundits.

Besides this, there is a rather significant disanalogy between race and religion.  Kennedy could cease at any time to be Catholic (and, if the gossip is true, he ceased quite often and with different women), Obama cannot at any moment cease to be black.  No amount of swearing on the Bible will lay to rest fears that he’s going to continue to be black. What is Obama supposed to say?  "I’m not, you know (wink wink), one of those people"?