Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election这里的it 为什么存在指代不清的问题
it是单数代词,前面句子中可以被指代的名词只有spending 和 the presidential campaign
但这两个做先行词都是讲不通/不合适,所以it在这里不行
最好直接就是"... a greater proportion than in any previous election"
这样的话 简洁而且comparison也清晰