Mel Martinez carried Miami-Dade County by 1,385 votes. http://www.cnn.com/ELECTION/2004//pages/results/states/FL/S/01/county.001.html
That may sound insignificant, but John Kerry beat President Bush in Miami-Dade County by over 48,000 votes. http://uselectionatlas.org/USPRESIDENT/statesub.php?year=2004&fips=12086&f=1
Mel's performance in Miami was instrumental in his statewide victory.
For the record, I agree that McCollum would have lost to Castor, but am not sure whether Harris would have won.
I stand corrected.