这句话的that是引导同位语从句吗?

题目

这句话的that是引导同位语从句吗?
When reformers in the northern states put pressure on Congress not to permit slavery in western territories ( that) later became states,some of the southern states wanted to secede,or withdraw,from the United States.
这里的that引导的 later became states是同位语从句吗?
还是引导定语从句呢？感觉应该是定语从句，that 在定语从句中作主语。

答案