More context would be nice. Can you link the article you got this from? Even without it, I can explain with a high chance of success, I think.
1. "being closed out of" can mean that he was asked to leave the office or that he was never allowed to enter at all.
2. Bill Cassidy is the Louisiana Republican. Within the story, we can guess this because the first quote came from someone who said "back in El Salvador" which implies that he is from El Salvador, and likely not a Louisiana Republican based on that fact. Also, a quick search on the Internet showed me that Bill Cassidy is a Republican in Louisiana.
3. "targeted by" means that, because Bill Cassidy opposed the immigration reform, advocates for the reform (people who support it) have attacked Bill Cassidy for opposing it. "Attack" here does not mean physical. They have likely tried to generate negative publicity (to get the public to hate Cassidy).
I'm not sure who the person from El Salvador is because the bit you quoted doesn't include enough information about him.