I'm pretty sure that the "senator" you are naming was Congressman Steve Scalise. I have not specifically heard that the shooter, James Hodgkinson, believed in conspiracy theories about a stolen 2016 election. But he may have.
However the big difference between partisan beliefs about the two elections is this. After 2016, Democrats mostly believed that they lost at the polls due to an effective disinformation campaign run by the Russians. After 2020, Russians believed that they lost due to widespread vote counting fraud. As https://www.rollcall.com/2021/02/24/partisan-voters-claim-we... says, Democrats and Republicans believe this by almost exactly the same margins.
But what is believed matters. Democrats may have been furious, but they believed in rule of law. Radical Republicans, by contrast, attempted to overturn the election via an insurrection.
However the big difference between partisan beliefs about the two elections is this. After 2016, Democrats mostly believed that they lost at the polls due to an effective disinformation campaign run by the Russians. After 2020, Russians believed that they lost due to widespread vote counting fraud. As https://www.rollcall.com/2021/02/24/partisan-voters-claim-we... says, Democrats and Republicans believe this by almost exactly the same margins.
But what is believed matters. Democrats may have been furious, but they believed in rule of law. Radical Republicans, by contrast, attempted to overturn the election via an insurrection.