Although a congratulatory call to the winner from his/her opponent is not a law, it has been a tradition since 1896
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15 Most Memorable Presidential Concession Speeches Over the Years (Videos)
The rocky road to the White House doesn’t end until that congratulatory call to the winner from his/her opponent
Media organizations[3][4][5] later analyzed the ballots and found that, under any considered criteria, the originally pursued, limited county-based recounts would have confirmed a Bush victory, whereas a state-wide recount would have revealed a Gore victory. Florida later changed to new voting machines to avoid punch cards, which had produced dimpled or hanging chads.