From the perspective of the Democrats, there are two reasons to have a comprehensive investigation of the riot: first, to identify the reasons for the lapses in security, and to fix them; and second, to develop facts that will cause the public to view large portions of the GOP as being complicit. The GOP leadership naturally opposes the latter; hence, their refusal to cooperate in the process.
But they cannot avoid it. If the Republicans boycott the proceedings and go on Fox every night to complain about how the system is rigged against them, they will only prove to everyone outside the base that they are, in fact, Trump’s accomplices, and a continuing danger to American liberal democracy.
That is exactly what I expect them to do. It will come back to bite them in some of the swing House districts next year.