My mother-in-law often tells me that "voting Republican is voting against myself."  She's a lifelong progressive who believes that Republicans are evil by nature (though considering that she's also an atheist, it strikes me as being philosophically problematic that she even believes in absolute good and evil).

Below is the answer I would give her if she would allow me (which she doesn't):

Public-sector employees get salaried with our money (via taxes).  Part of the public employees' salaries go to their unions as dues.  A good chunk of those dues goes to the coffers of Democrat politicians to fund their future campaigns.

See the cycle?  It is essentially money-laundering. 

Now, when a public union negotiates for more money or benefits for its members, and a Democrat politician is sitting on the other side of the negotiation table advocating for the taxpayer, who's going to lose?  Nine times out of ten...the taxpayer.  This must be the way of things for the cycle to continue.

By giving the union what it wants, the Democratic politician will ensure he (or she) gets what he wants.  The taxpayer will always be made to pay more to keep the cycle going. 

Union: If you don't do what we want, we won't give you campaign money.

Democrat politician: Raising taxes or allowing you to raise fees (like for public transportation) will be good for my next campaign.  If I need to go to the people, I will tell them that without the tax or fare hike, essential services will be cut.  That threat should scare them into giving us what we both want: more of their money.

Also, unions are a business like any other, with profit as their primary goal.  A union gets more profit by collecting more dues through more members.

Eventually, a tipping point is reached (often exacerbated by an economic downturn or an over-saturation of public employees -- think Greece).  Then, both public- and private-sector employees are in trouble.

But union trouble comes later rather than sooner, as the Democratic politician bleeds the taxpayer dry in an attempt to save the union from any economic troubles and continue the cycle.

In the end, the taxpayer is nothing more than a host for two sets of parasites working together.  Though a healthy host is required to keep the cycle going, the parasites are gluttonous creatures of habit which cannot stop their feeding -- even when it seems to be killing the host (think Obama's trillion-dollar stimulus).

And that, friends, is how a vote for a Democrat will always be a against yourself.  Questions?