First, I think the entire "liberal" v. "conservative" distinction does far more harm than good to productive public conversation. It causes people to pay more attention to where ideas come from rather than to the ideas themselves.

Second, it is a kind of ad hominem fallacy to claim that there exists some kind of unspecified (and no doubt nefarious) "liberal agenda" to which advocates subscribe. Especially given that 97% of the US supports universal background checks, how can the o.p. be so sure that this is the product of a "liberal agenda" and not instead a sensible response to an epidemic-level problem?

When people refuse to describe the entities they claim to exist (again: where and what is this "liberal agenda"?) there's good reason to be skeptical of both the people and the entities. Often it's scary packaging for something that's reasonable if you take the time to look at it.