The reach of any one person without an amplifier of sorts (traditional media, facebook popularity, etc), is essentially zero; Even in the extremely unlikely case that you've changed the minds of 1,000 people you talked to (because to change 1,000 people's mind and vote, you need to speak to 20,000), it's still a rounding error.
It will make you feel good that you tried while you're at it, yes. It will also demonstrate how powerless you are if the results go against your wishes (or falsely make you think you contributed if they go your way).
It's the same as fighting the centralization of the web, the big brothering of facebook/nsa, etc - you have to mobilize people to act, many of them. And sadly, it's very rarely possible or within one's power.
If you don't frame your life in a way that whoever wins matters to you very less, you are doing something wrong. If that is already the case, your vote doesn't count at all. It's one vote. The chance of your vote actually affecting the outcome of the election is so small that you might as well buy a lottery ticket instead.
If however, you have the power to convince a large enough group of people to sway the election to one side or another, you should sell some of that influence to whoever is offering more money or favors to you.
The answer is it does matter, and that you have such little influence is a failure of the system. I'm not talking about one vote, it's that some individuals have more influence over politicians due to their campaign donations. That is unacceptable.
The answer once again is get money out of politics.
You can only take money out of politics after you've taken money out of politics
The answer is not to suppress the leftist vote and care about the working class' concerns.