He raised $8.4 million in the second quarter and had $15 million in the bank as of June 30.
Harrison has massively outspent Graham in the race.
The Democrat's campaign spent $52 million on TV and digital advertising so far this year compared to $19 million for Graham, according to data from Advertising Analytics.
Between now and Election Day, Harrison has more than $16 million reserved, while Graham has $4.8 million.
Public and private polling has shown the race to be extremely competitive.
A poll from Quinnipiac University late last month showed the race tied at 48 percent.
An internal poll from the Democratic Senatorial Campaign Committee, conducted around the same time, showed Harrison narrowly ahead, 48 percent to 47 percent. »