Assumed Agreement Fallacy

One of the FallaciousArguments, wherein it is asserted that because noted expert X agrees with the speaker on some position Y, he likely agrees with the speaker on some other (often more controversial) position Z. Similar to ArgumentFromAuthority, but it's not entirely clear that the presumed authority even supports the position at issue--instead, it is made to appear that he/she does.

When Y is a single point, this fallacy is often easily refuted. When there are several Y's, however, that the speaker and the authority agree on--and taken as a whole, these points imply a general philosophical agreement between the speaker on the authority on some topic--it's much easier for the speaker to extrapolate agreement on Z when Z is a (likely) consequence of the (shared) general philosophy. Of course, Z is often--again--a more controversial point, so it still stands to reason that the authority doesn't necessarily agree with the speaker on Z.

In most cases, the authority isn't available to question, and has no published opinion on Z. Many noted authorities tend to shy away from taking strong positions on topics they haven't researched--even if it "generally" agrees with other beliefs they hold.

This works in reverse, too. A person may identify a point of agreement between the opponent and a group which is held in disrepute, and then imply that the opponent wishes to advance the nefarious agenda of the disreputable group. (See GodwinsLaw).