Don’t be so sure. Cardin is not a particularly distinguished Senator and at 75, a further demonstration of the aging Democratic leadership that it has to overcome.
Manning brings a story compelling to much of the Democratic base and those who aren’t compelled by it, will keep their mouths shut for fear of being branded bigots. The opportunity to elect not only the first transsexual Senator and gratuitously insult traditionally minded Americans may not be one the Dems would pass up.
They're not going to sacrifice the seat. Winning elections is far more important to Democrats than virtue signaling. Democrats understand better than Republicans do that the best way to effect change is by putting Democrats in office.
Republican primary voters haven't figured this out yet and still believe it is better that the guy you agree with 100% of the time loses than the guy you agree with only 90% of the time wins (hence the "Roy Moore for Senate" and now "Joe Arpaio for Senate" idiocy).