In the 2000 U.S. Presidential Election, Gore won the popular vote by 0.5% and lost the Electoral College vote.
In the 2008 U.S. Presidential Election, Obama won the popular vote by 7% and won the Electoral College vote.
In the 2012 U.S. Presidential Election, Obama won the popular vote by 4% and won the Electoral College vote.
In the 2016 U.S. Presidential Election, opinion polls forecasted Clinton win the popular vote by 3–5%. In reality she won by 2% and lost the Electoral College vote.
As of March 2020, nationwide opinion polls give either Sanders or Biden a margin of about 5% over Trump, only marginally more than the 2016 opinion polls just before election day gave Clinton, but more than Obamas 2016 margin.
Is there any rule of thumb by what margin a Democrat de facto needs to win in practice in order to also win the electoral college vote and become President?