how much damage are single issue voters doing to the democratic process and usa?

That's the problem with any form of democracy, including a republic. Single issue voters will vote for people/issues with very narrow interests in mind. The only hope is education, but of course talking about bettering one's knowledge-base and perspective on issues has become a sin (particularly among conservatives).

I would say some, but not more than any number of other negative influences, and certainly this is one we've dealt with for our entire history. Money in politics is a far bigger concern, in my opinion, and one that is much bigger than it used to be.

Single issue voters have always been around, whether they were voting for or against the power to create a national bank, the expansion of slavery, the gold and silver standard, prohibition, isolationism, communism, or any of the several we are dealing with today (many of which date back over a century). We've always managed a democracy working either with or around them, as the case may be.

Since when does the people's vote make any difference?

They tell everyone what they want to hear and go their own route once elected...they're ALL crooks.