Conversation so far:
What was Rick Santorum's tenure in regards to?
Rick Santorum served in the United States Senate representing Pennsylvania.
How long was Rick Santorum in the United States Senate representing Pennsylvania?
Rick Santorum served in the United States Senate representing Pennsylvania from 1995 to 2007.
Did Rick Santorum make a big impact in the United States Senate?
The National Taxpayers Union, a fiscal conservative organization, gave Rick Santorum an A− score for his votes on fiscal issues.

Question: Why did he leave?
Self-contained rewrite: Why did Rick Santorum leave the United States Senate?