当然不是that, it和that没区别
把a greater proportion带入
Soaring television costs accounted for a greater proportion in the presidential campaign of 1992 than in any previous election.
所以省略了soaring television costs accounted (they did0

谢谢回复！可是前后比较的是proportion，是否应该是a greater proportion than the proportion in any previous election,然后因为第二个proportion重复，所以省略掉

如果这么理解的话，形容词比较级后面是接的是比较对象，那I achieved a higher score this year than last year，那这句话的比较对象是score，难道省略了the score吗？明显不是，I achieved a higher score this year than (I achieved) last year，所以，这句话的比较对象不是proportion。
如果是比较对象proportion的话，应该这么表达the proportion in 1992 is greater than the proportion in any previous election。
纠正一下，本题省略的是soaring television costs have accounted for (they have done) ，in any previous election完成时标志

你给的这句话比较对象是时间呀