What happens if candidates declare victory in the 2020 election before results are final?

A candidate who declares victory before results are final could end up losing the election after all votes are counted. The election results published by state and local agencies on election night are unofficial tallies. After election night, results are finalized during what is called the canvassing and certification of the vote.

During this process, elections officials verify that votes were counted correctly. Officials review rejected ballots and finish tallying write-in, provisional, and mail-in ballots.[1] States then publish a report presenting the official election results. Often this report will include statistics on voter turnout, the number of absentee vs. in-person ballots counted, and the percentage of absentee ballots that were rejected. State laws set different deadlines for when each state must complete the certification of election results. In 2020, these deadlines range from mid-November to mid-December.
Several factors may contribute to delays in the reporting of unofficial election results in 2020:

• Changes to election laws and procedures due to the coronavirus pandemic have prompted lawsuits that could extend past Election Day.
• In some states, state law triggers a recount if a candidate leads by a small enough margin.
• According to one projection, voters will cast 80 million absentee/mail-in votes during the 2020 election, more than double the number cast in 2016. In some states, elections officials are legally obligated to wait until Election Day to process and/or count absentee votes. Moreover, some states accept absentee/mail-in ballots that are received after November 3.

As a result, many votes will be tallied after November 3, and we may not know the results of many races until after election night.[5] It is possible that a candidate who leads the race according to unofficial results on election night ultimately loses the election after all the votes are counted in the days and weeks after the election. For example, in Arizona’s 2018 Senate race, Martha McSally (R) led the race on election night, but ended up losing to Kyrsten Sinema (D) by 2.4 percentage points.

Information sourced from Ballotpedia.