<<I don't want every politician to have to apologise for being liked by odd people.>>
That's a fair statement as far as it goes, plane, but the problem is that Farrakhan is well beyond being "odd people." He's a racist with evil, hateful opinions of whites and Jews. A lot of people would consider him in the same category as David Duke. You pointed out yourself that Reagan renounced the support of a Klan leader.
The red line is drawn by where the endorser stands in the political spectrum - - it isn't as if it should depend on the "sensitivity" of the particular endorsee. It wasn't so much Reagan setting the standard by rejecting the endorsement of a Klan leader, the standard was set by the fact that the endorser was a Klan leader.