But bloody awful to whom, immigrants or the native born heritage population? The reason immigrants have almost always tended to support Democrats has primarily been due to:
A. First and foremost because immigrants are a large class of very needy supplicants and moochers. The party that empty's the public treasury to provide more free stuff is going to be the more popular one. Medicaid (Obamacare), higher education grants and loans, public education, federal housing, section 8, English as a second language, and all sorts of utility cost breaks and other breaks for them as "poor" is going to cause immigrants to align with the guy throwing funds from the public taxpayer at them.
B. Foreign immigrants are from societies with ideologies that rationalize and moralize mooching - fabian socialism, radical unionism, or generic nationalist left populism (e.g. Mexico). They are often of the lumpen proletariat, (not small businessmen).
C. Democrats has let it be known that the native white working class interests are secondary to the interests foreign citizens, legal and illegal. They have also staked out minority (and sex) identity as far more important class, and oppose the pressure to assimilate that might soften their tribal identity - pivotal to securing votes.
D. The only immigrants that tend to side with Republicans are those that have recently lived elsewhere under the ideology of the Democratic party to the extreme; i.e. the former citizen inmates living in communist regimes. Cubans, South Vietnamese, Venezuelans, etc. have historically understand the nature of the American left fellow-travelers, and don't feel nearly as compelled to become democrats as others.
Therefore, the Democratic Party (along with cheap labor Republicans) rabbit foot it to the throw the door open so as to stuff the national borders with people who will vote for them. Without immigration, this country would have remained center-right and most likely increasingly so.