I don't think they so much "allowed" it. I think it's just that the extreme elements are taking over the party. I would not be surprised at all to see the Democratic Party split at some future point. The party has been co-opted by the Hollyweird/NYC-Washington crowd. The Democrats have gotten by for decades because millions of lunch-pail, blue-collar union guys were brought up to hate Republicans. Believe it or not, Archie Bunker of "All In The Family" fame was a die-hard Democrat! Today, Archie Bunker would be a Republican. For now, many working class folk are starting to realize that the Democratic party of FDR, Truman and JFK is no more.
Behind his back, they refer to him as "The ATM" (machine).
It is not so much a matter of his bad image, or that he will polarize the party or make daily gaffes. It's that they believe he can bring a lot of money in.
Ka-ching.