In a nation where gerrymandering is a tradition and an art form, it is highly unlikely that Democrats will vote in favor of a Charter that reduces their representation in county government from their traditional 1/3 to a mere 20%.
If the Charter called for six commissioners with two coming from another a minority party, this would not be an issue.
Of course Democrats are significantly outnumbered by Republicans in the county. But it is unlikely that Republicans will be nearly as monolithic in their support as will single issue Democrats be in opposition. And that likely will tip the scale for defeat.