Identifying Ad Hominem Fallacy

In a recent debate about renewable energy, a speaker claimed, "If we start using solar power, then everyone will stop depending on fossil fuels. However, just look at the companies that are trying to undermine our efforts to switch! They're the same ones funding misinformation campaigns against renewable energy sources!" This argument appears to suggest that the opposition's interests discredit their claims about solar energy.

Identify the logical fallacy present in the speaker's argument above.