Post election polling strongly indicates that it was Greens who gave Al Gore his margin of victory in Florida (before he pissed it away). The Greens set up voteswap exchanges, and many Nader supporters dutifully turned out to vote for Gore in FL in exchange for Gore supporters voting for Nader in safe states, and most of those Gore-voting Greens would not have voted for Gore if Nader had not been in the race. So not only did Greens provide Gore his margin of victory, they handed the Dems a working model of how the two parties could cooperate to mutual advantage, and avoid the spoiler effect. It seems like this should have been a particularly appealing option to the DNC Dems because this offered a way to pick up Green votes where they needed them (at the cost of Dem votes where they didn’t need them) without having to make any shift in policy towards accommodating the values and preferences of Greens.
So how did the Dems respond? Not only did they not express gratitude for the Greens help in FL, or even acknowledge the help, they backhanded Greens with the outrageous false accusation that they were really the ones responsible for Gore’s loss, and then went to court to try to get voteswap exchanges declared illegal (unsuccessfully, so the option is still available to them–if they can ever scrape together enough working neurons to grasp how much they could benefit from them).
There may come an election where Greens strip off enough disaffected Dem voters to become a spoiler, but if that ever happens, 100% of the blame for that will be on the Dems. The Greens showed them how that could be avoided, and did all the heavy lifting in trying to build a bridge of cooperation between the two parties, and the Dems responded with Molotov cocktails. If that comes back to burn them later, it will be no worse than what they deserve.