Pursuant to 3 U.S.C. sec. 21, "election day" -- i.e., the date by which presidential Electors are chosen -- is defined to mean "the Tuesday next after the first Monday in November, in every fourth year succeeding every election of a President and Vice President held in each State, except, in the case of a State that appoints electors by popular vote, if the State modifies the period of voting, as necessitated by force majeure events that are extraordinary and catastrophic, as provided under laws of the State enacted prior to such day, 'election day' shall include the modified period of voting."
Accordingly, "the Dems" -- whomever you mean by that, although presumably you mean the Dems in Congress -- cannot, as a matter of federal law, "postpone the election." Individual state are afforded some discretion in modifying the date of "election day" in their own state (in extraordinary circumstances), but Congress itself cannot unilaterally act in the manner you are presumably postulating.
In truth, there is no single "Presidential election," in terms of the choosing of Electors. Rather, there are 51 separate such Presidential elections held every four years.
Stay tuned.