I need a name for a logical fallacy

[Fortinbras Armstrong]

The fallacy is because Person <X> agrees with Person <Y> on Subject <A>, they must also agree on Subject <B>.

I came across this one in a discussion with a libertarian, when I told him that the first socialized medicine program was instituted by Otto von Bismarck, in part because he agreed with the Socialists that socialized medicine would be good for Germany. The libertarian then said that Bismarck must have been a socialist.

[Galileo126]

1. This link might help

http://www.nizkor.org/features/fallacies/

[Fortinbras Armstrong]

3. I looked there, thanks

I also went to a few other sites -- TV Tropes, of all places, had a good list.

I might call it a hasty generalization, or perhaps an overgeneralization, but I'd really like something more specific.

[rrneck]

2. Sounds like Affirming the consequent

But I'm not much of a logician.

http://en.wikipedia.org/wiki/Affirming_the_consequent

Affirming the consequent, sometimes called converse error or fallacy of the converse, is a formal fallacy of inferring the converse from the original statement. The corresponding argument has the general form:

If P, then Q.
Q.
Therefore, P.

An argument of this form is invalid, i.e., the conclusion can be false even when statements 1 and 2 are true. Since P was never asserted as the only sufficient condition for Q, other factors could account for Q (while P was false).[1]

To put it differently, if P implies Q, the only inference that can be made is non-Q implies non-P. (Non-P and non-Q designate the opposite propositions to P and Q.) Symbolically:

(P ⇒ Q) ? (non-Q ⇒ non-P)

The name affirming the consequent derives from the premise Q, which affirms the "then" clause of the conditional premise.

[kairos12]

4. Bismark was a political realist. He needed a pressure relief valve

from the social pressures that were sweeping Europe. He used his socialized medicine program as a way to do this. It makes him a pragmatist, not a socialist.