IMHO, as a native Californian (for seven decades), I do NOT think that the Democrats are consciously working for Mexico.
BUT Mexican Americans (or Chicanos, as they were once known) are still very sensitive about their past treatment.
Until the 1970s, most Mexican Americans in Los Angeles, for example, lived in a few well-defined areas.
There were relatively few.
In fact, this month they are celebrating 1968, when Mexican-American students walked out of local high schools in order to demand better education and the end of discrimination against Mexican Americans.
*****
In 2018, the situation is 100% different.
1. Los Angeles is now a predominately Hispanic city (Mexican Americans + Mexicans + Central Americans).
2. Many of those "kids" in 1968 (or their children) now hold important city and state posts. (The attorney-general of California is a Hispanic American.)
3. Helped by their liberal friends (of all ethnicities), Hispanic Americans are now coming into their own in the Golden State.
NO, those Hispanic-American pols are NOT working for Mexico,. but YES -- IMHO -- they are enjoying doing everything possible to help "their" people, whose time they correctly feel has finally come.
Some years back,. an angry Hispanic-American pol had this advice: If you do not like the demographic changes in the Golden State, then leave! (And some people are, indeed, following his advice.)