But don’t the two go hand in hand?

If your voters are positively partisan, you run a positive campaign. If your voters are negatively partisan, you run a negative campaign.

No, you can run the negative campaign in either environment.

If the electorate is characterized by positive partisans, you run negative ads against your opponent to try to discourage their base. Your opponent's partisans are allied with the opposing party because they feel positively about it, so a negative campaign can reduce their affinity for the opposing party by attacking their candidate.

If the electorate is characterized by negative partisans, you run negative ads against your opponent to try to activate your base. The negative ads are still useful, but now they're activating the negative partisans on your team rather than going after the positive partisans on the opposing side.

And of course, negative ads are always useful against the unaffiliated or undecided. I don't say "independent," because a huge number of independents are not undecided. That's where a lot of the most intensely negative partisans are - people who loathe one of the parties, but don't have strong enough positive feelings with the other party to register or self-identify as belonging to that party. These are the people who hate Democrats or Republicans (either their politicians or their policies), so they'll never really consider voting for them - but don't bother joining the "other team," so to speak.