Why are many Democrats voting against deporting violent illegal migrants?

[AgnosticBoy]

This story came across my news feed from an article in Newsweek:

A majority of House Democrats voted against a bill put forth by GOP lawmakers that would require federal law enforcement to detain migrants who have entered the U.S. illegally and face charges for assaulting police officers.

The bill, which is titled the "Detain and Deport Illegal Aliens Who Assault Cops Act," was first introduced by New Jersey Representative Jeff Van Drew and backed by several other Republicans. It is among a series of legislation that the House GOP has vowed to bring to the chamber's floor in the coming days in honor of National Police Week. Republicans said in a release on Wednesday that the bills "highlight our continued support for America's law enforcement offices."

The measure passed the House in a 265-148 vote Wednesday; 211 Republicans were joined by 54 Democrats to send the bill to the Senate. All no votes were cast by Democrats. Six Republicans and 11 Democrats did not vote on the bill.

Full List of Democrats Who Voted Against the Bill...​

My initial thoughts are that these Democrats probably are against immigrants committing violent crime, but they voted against this bill for some other reason. Or maybe this article is leaving out some facts?... Just my guesses but I'll offer it up for debate.

I won't bother asking if assaulting police officers is wrong.

For debate:
1. Should we deport illegal immigrants that attack police officers?
2. Why did a group of Democrats vote against a bill that would deport immigrants that attack cops?