How much of an issue would a candidate legally buying and using marijuana be?

[Nichlemn]

If it had occurred very recently at the time it was discovered (i.e. it's not a youthful indiscretion), it would be a pretty big negative for any mainstream politician. There's a big difference between thinking something shouldn't be illegal and condoning your elected leaders doing it. There are tons of things that few people think should be illegal (like adultery, excessive alcohol consumption and atheism) that are nonetheless big negatives for almost any politician. I would imagine that a fairly large proportion of the people who think marijuana should be legal would nonetheless frown at a politician getting high, especially if they were running for President. Like if it were revealed that Hillary Clinton had smoked pot this year, it would really really hurt her "competency" image.