One candidate declaring victory has no legal impact. The losing candidate can concede defeat but again that has no legal effect (and candidates have withdrawn concessions when vote counts become more interesting).
AFAIK, States complete their vote counts in accordance with the rules in each State anyway. Remember that the ballot includes contests for House and Senate contests as well as those for the state legislatures and perhaps legislative initiatives, so no state is going to be able to save much time or money if it stops counting votes in the Presidential election. The most important impact of a concession is that it means the conceding candidate is closing the door on potential court challenges to overturn the vote count or requests for recounts (though, again, a candidate can withdraw the concession and challenge the result if s/he wishes).
Where it might create an issue is if the Supreme Court gets the opportunity to suppress the vote. If Trump gets an early lead and is able to incite rioting against further vote counting, pressure would go onto Biden to “do the right thing” for the country by accepting defeat and the Supreme Court might come up with some fiction to allow it to shut down counting to bring an end to the violence. Or a mob might storm a vote count to disrupt further counting and destroy ballots that are yet to be counted.
More importantly, Trump railing against the vote in places like Pennsylvania may give cover to the Republican legislature there to send a slate of Trump electors to the Electoral College competing with those won by Biden if Biden wins in that State. That might throw it back to Congress and could create an opening to steal the election.
PS: If it seems paranoid to suggest the Pennsylvania legislature might act in this way, consider that a month ago it tried to set up a standing commission with subpoena powers into voting fraud at the upcoming election. That caused an outcry from critics who thought it was laying the groundwork to reject the vote. The legislature was shocked, SHOCKED, that it’s laudable concern for election security had been misconstrued and deferred the issue until after the election.
AFAIK, States complete their vote counts in accordance with the rules in each State anyway. Remember that the ballot includes contests for House and Senate contests as well as those for the state legislatures and perhaps legislative initiatives, so no state is going to be able to save much time or money if it stops counting votes in the Presidential election. The most important impact of a concession is that it means the conceding candidate is closing the door on potential court challenges to overturn the vote count or requests for recounts (though, again, a candidate can withdraw the concession and challenge the result if s/he wishes).
Where it might create an issue is if the Supreme Court gets the opportunity to suppress the vote. If Trump gets an early lead and is able to incite rioting against further vote counting, pressure would go onto Biden to “do the right thing” for the country by accepting defeat and the Supreme Court might come up with some fiction to allow it to shut down counting to bring an end to the violence. Or a mob might storm a vote count to disrupt further counting and destroy ballots that are yet to be counted.
More importantly, Trump railing against the vote in places like Pennsylvania may give cover to the Republican legislature there to send a slate of Trump electors to the Electoral College competing with those won by Biden if Biden wins in that State. That might throw it back to Congress and could create an opening to steal the election.
PS: If it seems paranoid to suggest the Pennsylvania legislature might act in this way, consider that a month ago it tried to set up a standing commission with subpoena powers into voting fraud at the upcoming election. That caused an outcry from critics who thought it was laying the groundwork to reject the vote. The legislature was shocked, SHOCKED, that it’s laudable concern for election security had been misconstrued and deferred the issue until after the election.