as I recall, other dem candidates were campaigning/fundraising in Mexico last year.
Democrat Cory Booker Campaigns in Mexico, Hugs Babies and Walks Previously Deported Illegal Aliens Across US Border in El Paso
https://www.thegatewaypundit.com/2019/07/democrat-cory-booker-campaigns-in-mexico-walks-previously-deported-illegal-aliens-across-us-border-in-el-paso-video/
Beto Campaigns for President — Among Migrants in Mexico
https://www.lifezette.com/2019/07/beto-campaigns-president-migrants-mexico/
This just seems to be the democrat way. If you can't summon support in your own country, go get it from someplace else.