today’s polarized political environment主语 makes谓语动词 such an occurrence宾语 unlikely宾补 absent clear and convincing evidence of major wrongdoing介词短语作原因状语。

本句理解的关键是absent在本句为介词，等于without。用without替换absent，句子的结构就很清楚了：

today’s polarized political environment makes such an occurrence unlikely without clear and convincing evidence of major wrongdoing.

由于缺乏明确的、令人信服的证据表明特朗普犯了严重错误，如今二极化的政治环境使得共和党人加入民主党阵营要求特朗普下台的情况不大可能发生。