Immigrants tend to vote/side for Democrats. The truth is, the Democrats use them as pawns to do their bidding... to elect them in office.
If Democrats were truly for "undocumented immigrants" they would want them to come into the country LEGALLY so that they aren't being exploited by being paid pennies on the dollar for the work they do.
If Republicans were truly for immigration reform, the Republican controlled congress (and president when Bush was in office) would have fixed it. They would be proactively searching and punishing people who hire and exploit illegal immigrants. They wouldn't talk about amnesty... or a path to citizenship after they already broke the law and came in illegally.