Politicians this year are polarizing the electorate, and not necessarily along party lines. Democrats who don't want to vote for another Clinton presidency -- "I'd vote for anyone but Hillary" -- but cannot stomach the Republican offering either, will try desperately to push another Democrat forward at the convention. Sanders will probably benefit from this "hate both the frontrunners" mentality.
There is no unity in either party because they can not make a decision on who should be the leader and on top of that they can not put aside their differences to get stuff done. This election is like a schoolyard pissing contest, nothing is ever going to get done unless everybody gets over themselves and decides to do what is right for the country instead of for themselves.
The DNC convention will be divided because, much like the GOP, the party is extremely fractured. While not to the extent of the GOP, democrats seem less than thrilled about having either Clinton or Bernie Sanders as its selected presidential candidate. What an election season this is turning out to be.
20,000 people really is not that high of a number compared to the millions who are ready to support Hillary. Hillary had enough votes to win the primaries, and those people will continue to support her. There will be some discord, but there will not be a divided Democratic National Convention.