The "Don't Vote Democrat" club appears to be made of two groups of people. The first, larger group consists of clueless dupes who delude themselves with the idea that some "third" party responding to the will of "da people" is a real possibility. It is not and never has been - third parties of any kind failed to attract popular support beyond single digit numbers, but this does not stop these folks from fantasizing about it. It is, in my view, an outgrowth of American-brand religiosity that borders on magic - if you pray hard enough it will happen.
The second, smaller group consists of the fifth column players whose role is to sabotage Democrat voting deep behind the front lines. To understand the role of this fifth column, it useful to consider a model of the bi-partisan system in the US stipulated by the Hoteling's law. In short, this law says that in competitive duopolies, the center of gravity for both sides will fall close to the center of gravity of the whole system. That is to say, positions of both sides will be closer to each other than to positions of fringe supporters on each side.
This model implies a two fold electoral strategy. One is frontal assault to control the center, the other one is the "fifth column" to sabotage the territory deeply behind the front lines where main electoral battles are fought. The frontal assault is intended to sway the "median voter" to vote for party A rather than party B, and consists mainly by portraying the position of each side as being closer that that occupied by the "median voter." This is poli-sci 101 and should not be a surprise to anyone with a brain.
However, the frontal tactic to control the center has little effect on guys deep behind the front lines - both the "core" supporters and fringe radical even further removed from the center. It is clear that these guys have no other place to go and no tactical maneuvering to control the center will sway them to vote for the other side. This is where the fifth column strategy comes into play. It purpose is to sabotage voting deep behind the enemy lines among the core and radical supporters who are already far away from the gravity center from voting, a phenomenon known as "splitting the vote."
The fifth column strategy typically operates on the principle of increasing transaction cost of voting. Voting is a low transaction cost - low benefit activity for individuals. I will not benefit if I vote or lose much if I don't, but it does not cost me much to go to a polling station and cast the ballot. Since my transaction cost is low, I may be inclined to vote for emotional reasons even though I know that I will not benefit from voting in any meaningful way. However, if my transaction cost goes up, I may decide not to vote for "my" side of bipartisan divide or not to vote at all.
To accomplish its goal, the fifth column of the "Do not vote Democrat" club creates a subterfuge to increase the real or perceived cost of voting Democrat among its core or far-left supporters. A common tactic, often used by the Republican storm troopers, is increasing the transaction cost of voting, e.g. by requiring proof of registration, ID, or by limiting physical access to polling stations. This tactic can be used by those whose affiliation with the Republican party is no secret to anyone. A second tactic is more subtle, however. It aims to convince potential Democrat voters that voting Democrat has some hidden cost, for example that Democrats are "sell-outs" "turn-coats" or "traitors" so voting for them will make things worse. To be effective, this tactic requires that the identity of the messenger is fake or at least concealed. This advice must come from someone who appears to be on "our" side to be taken seriously. Nobody would believe it if it came from a known Republican hack.
So here comes the fifth column of the "Do not vote Democrat" club on the left. Some of them may be false flag operators, republican functionaries posing as radical lefties. But most of them are probably not - more likely they are useful idiots who willy-nilly play into the hands of the Republicans. They may be genuinely disgusted with the politics of compromise, lack of proper zeal among Democrat politicians, or just the general way things are going. They may be swayed by the idealism of the first group in the club who believer that this country is a populist democracy and casting votes is all that it takes to change the deeply ingrained institutional structures. Or they may be waiting for a savior and when someone who looks like one appears, they may put all their hopes in him, and if those hopes fail to materialize, they become disillusioned and bitter and demand a crucifixion of the "fake savior."
Whatever their motivation, their passion and their convictions are genuine and that is what makes them perfect fifth columnists. Nobody will suspect them of being a false flag operation for the enemy. They are genuine radicals sincerely fighting for the cause. The fact that their efforts help the enemy does not typically enter their consciousness where ideological purity and moral convictions reign supreme and reason has been relegated to the periphery where it is only a tiny bit away from treason.