WASHINGTON — Just days before the presidential election, the chief executives of three leading technology firms — Jack Dorsey of Twitter, Mark Zuckerberg of Facebook and Sundar Pichai of Alphabet, Google’s parent company — testified before the Senate Commerce, Science, and Transportation Committee on their efforts to combat misinformation and prevent bias from taking root in their content-moderation efforts.

Few sectors of the American economy face as much opprobrium from both the political left and right as does Silicon Valley. Republicans, in particular, believe that Big Tech is inherently biased against conservatism because technology companies are stocked with progressive employees. That conviction formed the basis of the hearing — and led to some heated exchanges.

A discomfiting back-and-forth about Holocaust denial was symbolic of how fraught the issues surrounding social media are, and how Republicans increasingly believe that Big Tech is using extant laws to engage in political censorship that favors progressives and, in some cases, left-leaning extremists.