How will this allegation affect the re-election chances Trump has?
Probably as much as the Tara Reade allegation that Joe Biden has levied against him; A whopping zero. There's been dozens of sexual assault allegations against Trump which haven't produced any evidence. If there's any sort of evidence against him that is released, that's when people, including me, will care.
Joe Biden is in the same corner. You have the Tara Reade allegation (I did a detailed write up here which is a fair assessment imo
https://bitcointalk.org/index.php?topic=5237990.msg54406308#msg54406308). You also have a few women who've came forward accusing Joe Biden of touching them inappropriately, who Kamala Harris has said she believes, yet that doesn't seem to sway people.
For some odd reason, all of our old politicians are getting Me Too'd. Maybe because we live in different times, but I guess back in the 20th century, middle aged men treated women like shit and now all that baggage is following them in modern times, OR, some of these women are lying for political reasons. I think both might be true depending on the individual case. You have too many people that are retroactively making sexual assault claims after their alleged perpetrators seek higher office despite being in the public eye for ages. Same thing with Justice Kavanaugh and Christine Blasey Ford.
Bernie Sanders might be the only politician that's old and doesn't have any skeletons in the closet regarding sexual assault left in Washington D.C.