The South Carolina senator described Harrison as a "formidable" rival in a series of congratulatory messages shared to Twitter on Thursday.
The South Carolina Democrat pulled in a colossal sum as he challenged Sen. Lindsey Graham, and is now turning his sights to backing candidates in Georgia.
Jaime Harrison previously served as the chair of the South Carolina Democratic Party before running his ultimately unsuccessful Senate campaign against Republican incumbent Lindsey Graham.
The Democrat conceded to the incumbent Republican Lindsey Graham and suggested President Donald Trump should follow his example.
"All the liberals in California in New York, you wasted a lot of money," Sen. Lindsey Graham said after defeating Democratic challenger Jaime Harrison in a Senate race that included over $200 million in funding.
The incumbent's win in ruby red South Carolina was a surprisingly difficult victory for Republicans in the age of President Trump.
Harrison raised a record-breaking $131 million by October 14, prompting Graham to beg Fox News viewers for donations.
The incumbent has faced a tough campaign battle from Democratic contender Jaime Harrison who has broken election fundraising records.
The Democratic challenger has raised some $109 million compared to the Trump ally's $74 million.
Graham said Saturday that Twitter moderators believe the Department of Homeland Security is more of a threat than the ayatollah of Iran.
"I'm not sure you believe in *anything* except preserving yourself," Rep. Alexandria Ocasio-Cortez (D-N.Y.) tweeted to Sen. Lindsey Graham (R-S.C.) after he debated his Democratic challenger Jaime Harrison on Friday night.
The pair are due to go head-to-head on Friday evening amid a tight race for the Senate seat currently held by Graham in South Carolina.
The Democratic contender for South Carolina told "Late Show" host Stephen Colbert "Lindsey Graham's word isn't worth much of anything."
"Lindsey Graham, a political parasite with a record of lies and personal betrayals," the ad begins.
Harrison, former chair of the South Carolina Democratic Party, has gained national recognition for his fight to unseat Graham, who has served in the U.S. Senate since 2003.
The South Carolina Senate race is fast becoming one of the most expensive in the nation as both sides raise record-breaking sums.
"We started as a 17-point underdog. Today, I'm honored to accept the endorsement of the oldest newspaper in South Carolina, @thestate. Change is coming, folks," Harrison wrote on Twitter alongside a link to The State's endorsement, written by members of its editorial board.
The South Carolina Republican is still forecast to win, but his better-funded rival is gaining ground in the run up to the vote.
As a recent poll showed him leading the incumbent Republican senator by just two points, the Democratic candidate called for a $10 million donations goal.
Graham's Democrat rival raised $57 million in the third quarter, and said this had all been spent on campaigning already.
"Yes, we raised $57 million, but we spent $60 million," Harrison said. "But that is what we had to do in order to get this thing to toss-up status, in order to bridge the gap."
"Help me get my message out," the incumbent Republican from South Carolina said.
The poll, which was conducted by Morning Consult, found 47 percent of South Carolina respondents saying they'd vote for Harrison, while 45 percent sided with Graham.
"[Lindsey Graham] believes that he represents the interest in Washington, D.C. instead of representing the interests of the people in South Carolina," Jaime Harrison said Wednesday.
The Democratic challenger has opened up a one-point lead over the Republican incumbent with just weeks to go until polling day.
Critics say the South Carolina senator may have committed a crime with a televised plea for campaign donations.
South Carolina Senator Lindsey Graham later told reporters he made the comment "with dripping sarcasm."
Graham has served as a senator for the traditionally red state since 2003, but several polls have shown Harrison tied in popularity.
Despite falling behind in fundraising, Graham re-opened a polling lead over Harrison that he hasn't enjoyed in nearly a year.
Harrison raised $57 million from July through September, the highest quarterly fundraising for a Senate candidate in U.S. history.