WASHINGTON -- U.S. Sen. Johnny Isakson is sitting on nearly $4 million to ward off potential challengers for his re-election campaign next year.

The Georgia Republican's campaign announced Tuesday that he raised $1.6 million in the first three months of the year and has $3.75 million on hand.

By comparison, at this point in his last re-election cycle, Isakson raised just $379,000 and had $2.47 million on hand.

Seeking to combat persistent retirement rumors, Isakson, 70, got an early jump on the 2016 cycle by announcing his candidacy for a third term in November. He recently suffered three cracked ribs -- starting the rumor mill once again -- but continued to raise money and conduct official events.

Isakson has no formal challengers. Republican Stone Mountain minister and MARTA engineer Derrick Grayson, an also-ran in the 2014 Senate primary, has said he is exploring a run but has not reported any recent fundraising to the FEC.

In a statement via his campaign, Isakson said:

"I am proud that Georgians continue to feel that I effectively represent their interests in Washington. I look forward to this campaign and supporting all those conservative candidates across the country who are also dedicated to keeping our Republican majority and taking back the White House."