This also has Biden winning by 7%, so yet another poll showing Peters underperforming the Democratic nominee for president by 3 points.
We saw this a lot in 2016. Ross, Murphy, Duckworth, Feingold, Cortez-Masto, Barksdale, Hassan, Judge, Strickland, and McGinty all underperformed Hillary. Bayh, Gray, and Kander were the only ones to outperform her.
I bolded all the ones who were facing incumbents who weren't as scandal ridden as Roy Blunt and thereby had crossover appeal to "put a check on Hillary" voters, name recognition, time to build more goodwill in their respective states, tailor their stances to their respective states, et cetera.
It seems to be a common occurence for Senate (and House) incumbents who aren't scandal ridden and who don't face particularly strong opponents, to outperform the Presidential candidates from their parties in their respective states.
Take a look at what happened in 2008 and 2012 - quite a few incumbents outperformed Obama.