In a poll reported by Rasmussen Reports on Twitter, 30% of Democrats believed it was likely that Democrats committed election fraud:
Huge: "How likely is it that Democrats stole votes or destroyed pro-Trump ballots in several states to ensure that Biden would win?"
Democrats - 30% - 20% say Very Likely (VL)
Unaffiliated - 39% - 29% say VL
Republicans - 75% - 61% say VL
All Voters - 47% - 36% say VL
This doesn’t per se mean that these people believe that Trump won, as Biden getting the most votes and Democrats committing election fraud are not necessarily mutually exclusive. For example, you might think that several small occurrences of “stealing votes” happened, but nothing big enough to affect the results.
(I won’t address if any fraud happened here as it’s a different question. It may not even be a question that can be satisfactorily answered right now, considering that some of the convictions this year for election fraud were for crimes committed in the 2016 election.)
According to Rasmussen’s website, their poll was done November 17-18 2020 with a mixture of automated phone and online polling.
Opinion polls tend to vary in outcome depending on the wording of the questions and other, hard-to-pin-down factors. Rasmussen Reports, according to several sources cited by Wikipedia has mixed reviews for how accurate it is, and also is said to have a Republican bias. Another poll, from a source that Wikipedia speaks favorably of (despite failing to predict Trump winning in 2016), put ‘the share of registered voters who say they trust the United States' election system either "a lot" or "some"’ at 80% for Democrats on almost the same day (November 16th). And other numbers were higher, such as the 91% of Democrats falling into ‘the share of registered voters who say the 2020 presidential election was “probably” or “definitely” free and fair’.
(Unfortunately I haven’t been able to find anything that directly asked if people believe Trump lost the election due to fraud.)