postal ballots do more harm than good
This is not an “either or” topic which means you do not need to prove that one side is entirely true while the other is entirely false. You simply need to prove that your side is MORE true than the other side.
Historically the use of postal ballots have helped the Democratic Party more than the Republican Party and since more people are expected to vote by mail this year than ever before, there are concerns surrounding the legitimacy of the process.
Regardless of which side of the topic you are arguing on, it is a smart strategy to acknowledge some disadvantages your opponent is sure to bring up. After all, your side may not be perfect but it is better than the alternative.