He's trying to tell people who live in Open Primaries states to go out and vote for Bernie in the Dem. Primaries.
The Dems. have done it in the past in Republican Primaries, so why shouldn't the Repubs. do it to the Dems. ? 15 Open Primary States. South Carolina is one of them.
There are 15 states that allow voters to privately selected which primaries in which to participate. A registered Democrat, for example, could choose to cross party lines and vote for a Republican candidate. "Critics argue that the open primary dilutes the parties’ ability to nominate. Supporters say this system gives voters maximal flexibility—allowing them to cross party lines—and maintains their privacy," according to the National Conference of State Legislatures.
Those 15 states are:
Alabama
Arkansas
Georgia
Hawaii
Michigan
Minnesota
Mississippi
Missouri
Montana
North Dakota
South Carolina
Texas
Vermont
Virginia
Wisconsin
https://www.thoughtco.com/what-is-an-open-primary-3367495