Once you have been trained to believe 57 different lies and fabrications, believing one more is quite easy.
And it isn’t even necessary to believe them all. The sheer volume of garbage that is spewed at the various targets tends to stick, even among skeptics. The volume of filth that was and still is directed at Clinton and Obama was so incredibly high that the simple fact of the volume affected many people’s judgement of these people, even if they didn’t believe any individual piece. That’s the purpose of the efforts to dump massive amounts of filth on those that the Trump camp fear. The more they fear them, the greater the volume of filth they spew.
The one exception is Putin. I think they are terrified of him and what he can do to them, so they will brook not the slightest hint of dissent about Putin. Anyone else is fair game, and the more they fear them, the crazier the diarrhea they stream at the target.
Even today, if President Obama broke protocol and really took Trump apart, in a concerted effort over a few months, Trump would be in a mental hospital by the end of it. Had Clinton been elected, there would have been some seriously hard sanctions on Russia and Putin’s assets would have been systematically stripped from him outside of Russia. Comey, McCabe, Mueller and Yates were/are all onto the Trump organization and were in a position to close in on them, so had to be eliminated and buried in filth as part of the process.
Once you believe even a part of this stuff, you are sufficiently credulous to believe anything. As the flat-Earth doctrine is a deliberate confidence trick from its first inception in the late 1840s, it’s just made to suck in the gullible. And Trump’s supporter are all that and more.