Dr. David Kelly was almost certainly assassinated by "our" side. For those for whom its a bit before their time he was one of the lead weapons inspectors looking for WMD in Iraq. He had leaked to the Guardian that he'd found insufficient evidence and once he was exposed as the source, his card was marked. The reason being that the entire legal basis for the UK attacking Iraq depended on WMD. His testimony could have actually derailed the UK case for war. It is absurd to think that the same people who will knowingly take a course of action leading to hundreds of thousands of deaths in a foreign country on a pretext they know is false, would suddenly balk at one more body. Seth Rich certainly assassinated. Epstein, obviously. It's purely a matter of practicality and necessity.
So lets assess it in such terms. I'd say there is a qualified possibility. The qualified is that I don't think "Left" is a good term for this. We're talking specifically the entrenched Establishment which is not quite the same as the "Left" is, just as "Progressives" are not wholly the same thing as the Left.
That qualification out of the way I'd say it has a low but real chance. I think the play they will try to stick with is lawfare. Imprison him, try to ruin him financially, personal attacks and social exclusion to try and put pressure on his family and home life... IF that doesn't work and he looks like he's heading for candidacy as president there is still the play to "fortify" the electoral process again. That is high risk, especially after last time. But it is probably still less risky than actually trying to kill him which is a huge gamble. Not least because he still I presume has Secret Service protection and those guys are the best there is. I don't think easily suborned either, hopefully.
But in the scenario where all these measures have failed and he is going to be president unless they target him physically, yes, I wouldn't wholly rule it out. And for the comments about this being an age of media, PR fall-out, etc., it's also an age of incredibly sophisticated medical knowledge and technology. There are numerous ways they can kill someone that are very hard to detect. Hell, there's numerous ways you they can just shatter someone's health to the point someone is no longer fit for the role.
But on the downside (for them), the risk is through the roof. Try and fail, you've just handed him the election and also made his number one priority - and that of the US public - to root out the people behind the attempt. Try successfully, and nobody will believe natural causes.
There's also the decreasing faith that the Establishment can protect the assassins. With Seth Rich, whoever killed him could have fairly high confidence that the Clintons could prevent him being investigated or even found. Now, much more is up in the air and there's less certainty that those promising "it's okay, we'll protect you," actually can.
However, these downsides are balanced by necessity. If Trump gets in he's going to tear down a lot of their entrenched powerbase, derail a lot of their plans, he's a wrecking ball for them. A disaster. He was constrained last time by not knowing how Washington worked up close, by needing to play nice to get a second term, by trusting the wrong people. This time he'll go full Augean Stables with DC. And they fear that greatly.
Summary of my take: They have other measures which may work and it may not get to this point. If it did, I'd still only give it 1 in 3. So... I think there could be an attempt, but probably wont be.
They'll assassinate the American economy instead. They are already playing hot potato with the economy for the last 4 years, if Trump wins they just detonate it and blame him for the fallout. Same way they remove his economic achievement through COVID.
Well yes, there's a massive shift in the US economy on its way. It's in the plans. It's quite plausible that they would prefer to hand the giant mess over to the Republicans this term and let them take the fall-out. BUT, they want a uniparty Republican in place. Like a Desantis who will take the hit nobly and follow the gameplan for dealing with the exploding economy. If the storm hits (which it's going to) and Trump is at the helm, who knows what he might do to deal with it. Not the Democrats, that's for sure.