I don't think they actually believe it.
Like so many personal accusations, they think there is some political juice to squeeze here.
It will have not have any impact on the 2026 midterms.
However, it does satisfy the progressive base's need to feel like they are fighting Trump.
Shaping some popular policies might work better. The problem is that not only requires appealing to the electorate, but also building a consensus within the Democratic party.
Lets see what we do know:
Epstein definitely was a pedo predator
Epstein island exists and the Lolita Express was real
Epstein was charged and likely killed in jail.
Maxwell was a willing partner and found guilty of trafficking children
Maxwell did give politically motivated testimony in exchange for a lenient sentence or parole
Multiple witnesses came forward and testified that trump and others were clients
The trump./Epstein connections are multiple and well documented
There were charges pressed against trump/Epstein for raping a child that were dropped after the victim reported death threats
trump has a long history of rape allegations and charges, including being adjudicated as a rapist and fined $83 million for slandering a victim
trump has a long documented history of paying off women
We don't have the video that Maxwell claimed exists but we do have multiple allegations from victims that are all credible.
Given this evidence a morally responsible person would say that they wouldn't support someone unless they were clearly exonerated on all cases.