This is a much larger question that what should happen with undocumented workers,or how best to secure the border请划一下成分

This 主语
is 系动词
a much larger question 表语
that what should happen with undocumented workers,or how best to secure the border question的同位语从句，其中包括两个独立的句子，用or分隔。从句用以描述问题（question）是什么
lz注意that后的句子不是定语从句，因为定语从句要求先行词（question）在从句中充当某种成分

哎！一看就是中国人写的。
This是中国人的用法，最好用It
larger question 我看懂了，是大问题
可是larger修饰问题，确实有点...中国人能看懂，老外就会糊涂。
question that后面是主语从句：
应该是:
What should happen with undocumented workers,or how best to secure the border is a much larger question.

四楼更正的好。

This is a much larger question /that what should happen with undocumented workers,or how best to secure the border。
后面的是宾语从句，修饰的是question

我查了一下原文,您的这句话是错误的,应该是That is a much larger question than what should happen with undocumented workers,or how best to secure the border,and it is one that affects not only newcomers but groups that have been here for generations.It will have more impact on our future than where we decide to set the admissions bar for the latest ware of would-be Americans.And it would be nice if we finally got the answer right.

question 后的部分是定语从句 that从句做定语从句修饰question