The Atlanta-San Francisco game already opened with the 49ers favored by 3 points, but heavy early money on San Francisco has driven the line to 4.5 points. According to RJ Bell of Pregame.com, that move now has Atlanta as the largest home underdog by any #1 seed since 1978, and the largest home underdog for any team in a championship game since 1978.
Home underdogs in the playoffs have a history of success (21-14 straight up, 22-13 against the spread), but that is mostly true in the wildcard and divisional round. In the 12 prior championship games where the home team was an underdog, the visitor won (and covered) 7 of 12 times.
[photo via USA Today Sports Images, data via pro-football-reference.com]