Even where there is no prospect of achieving their election the workers must put up their own candidates to preserve their independence, to gauge their own strength and to bring their revolutionary position and party standpoint to public attention. They must not be led astray by the empty phrases of the democrats, who will maintain that the workers’ candidates will split the democratic party and offer the forces of reaction the chance of victory. All such talk means, in the final analysis, that the proletariat is to be swindled. The progress which the proletarian party will make by operating independently in this way is infinitely more important than the disadvantages resulting from the presence of a few reactionaries in the representative body. If the forces of democracy take decisive, terroristic action against the reaction from the very beginning, the reactionary influence in the election will already have been destroyed.
I'm saying this in support of what Marx said, not against it. You're arguing with spooks.
The quote you posted is the essence of what "don't let perfect be the enemy of good" means.
Vote for good instead of letting the fact they don't have a perfect chance of winning stop you from supporting what is good.