These two conditions are sufficient:

(1) When the race is close or

(2) When the prize is big.

Federer can afford to relax in a match with small stakes or when he is close to the winning point.  But he should work hard at Wimdledon and if the match is tied.  The same principles apply in elections.  In the Massachusetts special election, it has been known for months that the second condition is satisfied.  Because, without the Massachusetts Senate seat, the 60 vote barrier needed to fight the filibuster is gone.  The White House, the Democratic Party etc should have been focused on the Massachusetts race on this ground alone.  Belatedly, it was discovered that condition (1) also obtained.  But even you didn’t know that, you did know that the coalition you had to hold together to get stuff through the Senate was pretty fragile and one crack was enough to send the whole thing flying.

The Republican Party recognized that the second condition was key, worked like crazy and played the optimal strategy.  It’s common sense but only one party seemed to get it.