Here are figures listing the teams with the lowest season and decade attendance for each 10-year period from the 1900s through the 2000s (most data taken from the Bill James Historical Baseball Abstract).
1900s:
Philadelphia Phillies, 1902, 112,066
Boston Braves, 1,492,753
1910s:
Brooklyn Dodgers, 1918, 83,831
Boston Braves, 2,088,310
1920s:
Boston Braves, 1920, 162,483
Boston Braves, 2,499,518
1930s:
St. Louis Browns, 1935, 80,922
St. Louis Browns, 1,184,076
1940s:
St. Louis Browns, 1941, 176,240
St. Louis Browns, 3,330,861
1950s:
St. Louis Browns, 1950, 247,131
Washington Senators, 5,598,081
1960s:
San Diego Padres, 1969, 512,970
Washington Senators, 5,834,745 (nine years)
1970s:
Oakland A’s, 1979, 306,763
Oakland A’s, 7,646,599
1980s:
Minnesota Twins, 1981, 469,090
Excluding 1981: Cleveland Indians, 1985, 655,181
Seattle Mariners, 9,839,630
1990s:
For the two strike seasons:
San Diego Padres, 1994, 953,857, and 1995, 1,041,805
For the other eight years:
Montreal Expos, 1999, 772,737
For the decade: Montreal Expos, 13,006,225
2000s:
Montreal Expos, 2001, 642,745
Florida Marlins, 13,477,204
A few comments: I don’t know how the Braves and Browns managed to stay in Boston and St. Louis for so long; even with the travel constraints of not having jet airplanes, which made the West a fairly unviable option, either team apparently could have moved to Atlanta, Columbus, Buffalo, Milwaukee, the Twin Cities, Toronto, or Baltimore at some point before World War II and seen a big attendance increase. And this is what happened when they did move in the ’50s.
Although attendance, even at the bottom of MLB, has pretty steadily increased from the ’40s onward, the A’s 1979 attendance stands out as a figure that wouldn’t be out of place in a Depression year.

It’s unbelievable that the A’s could win three consecutive World Series in the 1970′s, and still finish with the worst overall attendance for the decade. They should have been allowed to move years ago. Oakland would be a legitimate town for a Double-A team, not an MLB franchise.