You think the campaign trail is tough? Try sparring with a UFC champ!! That's what presidential candidate Tulsi Gabbard did in Boston last week -- trading shots with strawweight superstar Weili Zhang ... and TMZ Sports has the video. Remember, Gabbard played a huge role in helping the Chinese superstar get a visa so she could come to the U.S. on a media tour. Gabbard, a U.S. military combat vet and current member of the Army National Guard, is a HUGE fan of MMA and has watched Zhang compete for years. So, when the opportunity came up to spar with...