"Animal agriculture is responsible for 13–18% of human-caused greenhouse gas emissions globally, and less in developed countries (e.g. 3% in the USA). Fossil fuel combustion for energy and transportation is responsible for approximately 64% of human-caused greenhouse gas emissions globally, and more in developed countries (e.g. 80% in the USA)."
I think in the USA, it would be more productive to address the 80% as higher priority than reducing the 3%. I think this veganism initiative is also more driven by morality and political partisanship than by an effort to find a realistic and practical solution to GHG emissions. And this can backfire: I believe that a reason why nearly half the country will not even openly admit there is a problem, is because they fear the consequences of political over-reaction more than the problem itself.