This article cites a poll that indicates that “Forty-eight percent of Republicans said they believe between 3 million and 5 million people voted illegally in 2016.”
Even “Just under one-quarter of Democrats said they believed the allegations that millions of votes were cast illegally…”
The article links to a Quinnipiac poll from 2017 (question 78).
The poll found that 50% of Republicans, 24% of Independents, and 14% of Democrats believed there were 3-4 million illegal votes.
For more background on why this theory is nonsense, read this.
I am concerned that Trump will again claim there were illegal votes if the Democrats win the House and/or Senate this year. Trump has already said recently there were millions of illegal votes in California and there are false claims of illegal votes in Pennsylvania. These are the two states most critical for Democrats to gain seats in to win the House. I am afraid Trump will try to invalidate the results if the Democrats win. This is what his dictator pals would do.