I don't agree with this. Losing ground implies there is ground to lose. If there is ground to lose, that ground came from somewhere. We gained the "ground that we have to lose", under the same two-party system we have now. For example, slavery didn't abolish itself.
Think back to the 1950's. Back then you could have argued that voting is pointless because we're going to lose ground either way. But then we had the civil rights era and clean air / clean water act. People complain about trace water pollution these days that "might" cause disease or cancer, whereas decades ago raw toxic sludge was openly dumped into Americas rivers.
There was a war associated with abolishing slavery. Civil rights era also falls under monumental civil disobedience and revolutionary movements. Some environmental protections may have been passed decades ago, but they have not been adequate or have not prevented the political establishment from enabling environmental destruction to continue to increase. Besides, show me any candidate who will make environmental protections a priority now, and I will vote for them
enthusiastically.
And... you're not seriously telling me that environmental problems we face today aren't as serious as they were in the past, are you?
But even for ones with protests, do you think it makes no difference at all who you elect in those circumstances? The government has a number of ways to deal with protests. Appeasing them is a choice. The different parties approach dissent much differently. For example you're not going to see Republicans appease black protesters any time soon.
Yes, it does make a difference. But the candidates that will make the difference you want still have to be there.
But not a revolution. The pro-government side was the anti-slavery side, the rebels were the pro-slavery side, so it is the exact opposite of advancing a social right through revolution.
This is more complicated than you're making it out to be. It was about slavery for many people, but for the government, it was mostly about preserving the union.