The best way to counteract an opposing viewpoint is to anticipate what an opponent might say. When researching the topic, then, don’t limit yourself to sympathetic sources; find sources that disagree with your argument. Take note of their rationale and use of evidence. That way, you will be familiar enough with these opposing viewpoints to argue against them. When you encounter dissenting opinions, try to figure out why smart and rational people would hold those positions. What evidence do they look at? How do they interpret that evidence? Why might they disagree with your point of view?
When we’re passionate about a topic, emotions can sometimes cloud our rationality. We tend to have disdain for opposing arguments and aren’t open to even hearing what those on the other side have to say. To move yourself out of this emotional realm and back into the realm of the well-reasoned argument, try taking a strategy from debate tournaments. Debaters prepare for tournaments by gathering information on both sides of a topic. They actually don’t know which side they’ll be arguing until the debate begins, and so they must be just as prepared to argue the side they don’t agree with as the one they believe in. As you’re researching, then, take the debater’s approach to gathering information so you’ll be very well-informed about the opposing views.