For example, when I was a child, I knew full well that Coke was bigger than Pepsi and that McDonalds was bigger than Burger King, and that neither one really needed my loyalty or support. However, the Pepsi and Burger King commercials at the time were very negative and competitive in tone, always trying to prove that Coke and McDonalds were inferior and that people only used them because they were uninformed about the glorious alternatives. As a result, I developed a deep-seated resentment of both Pepsi and Burger King, to the extent that I only use either product when no other option is available. It doesn't matter that they're really #2 and that the products they were attacking are still #1 -- the mere act of attacking their competition made me feel as though their competition were the underdogs, the oppressed, and therefore worth sticking up for. And this feeling is still with me thirty years later.
The same applies, in an even more significant way, to political endorsements. Any argument based on slagging off the Other Guy, even if that Other Guy is the current top dog in the system, is going to have the opposite effect of the one you want -- it's going to make me defensive on his behalf. For one thing, I'm cynical about the integrity or reliability of politicians to begin with, so telling me that Politician A is a lying weasel is not going to surprise me. For another, I am also cynical about the integrity and reliability of the media, so any Shocking!News!Stories! you'd like to present as evidence of Politician A's calumny will be taken with a generous helping of salt.
In other words, my first reaction to your devastating indictment of the opposition is going to be "Oh yeah? And your politician/party hasn't behaved just as badly or worse?" while my second will be, "Assuming this stuff you're saying hasn't been blown way out of proportion/taken out of context/made up out of whole cloth to begin with."
If you want to actually impress me or have any chance of persuading me, you'll need to think positive, not negative. Instead of cutting down your opposition or complaining bitterly about the status quo, try telling me the benefits of seeing things from your perspective, and showing me all the good things your side has done or is in the process of doing. Try being courteous to your opposition, and giving them credit as intelligent human beings -- however misguided, erroneous, or even venal you may think they are -- instead of loathsome caricatures. Trust me, that'll go a lot farther to make me sympathetic to your point of view.