Former Florida governor Jeb Bush arrives at a campaign event at the Greasewood Flats Ranch in Carroll, Iowa, on Jan. 29.(Rick Wilking/Reuters)

The super PAC supporting former Florida governor Jeb Bush's Republican presidential campaign raised $15.1 million in the second half of 2015, bringing its total war chest for the year to $118 million.

The biggest donation to Right to Rise USA in the last six months was a $10 million infusion from a company owned by Maurice "Hank" Greenberg, former chief of American International Group. That makes Greenberg the largest donor to the super PAC, followed by Florida health-care executive Miguel B. Fernandez, who gave $3 million earlier in the year.

An additional $1 million came from Morton Bouchard of Melville, N.Y.

Right to Rise USA has plowed nearly $65 million of its haul into television spots, digital ads and mailers aimed at boosting Bush, making it by far the biggest independent spender on the 2016 presidential race. In a distant second is Conservative Solutions PAC, which has shelled out more than $20 million to support Sen. Marco Rubio (Fla.).

In recent weeks, Right to Rise has unleashed a slew of attacks against Rubio and other establishment-friendly GOP presidential candidates, despite criticism among some Republicans that it should aim its firepower at front-runner Donald Trump.

Still, even with the super PAC's substantial air support, Bush has struggled to gain traction. In Iowa, which will hold the first-in-the-nation caucuses Monday, Right to Rise has spent more than $14 million, but the former governor remains mired in the single digits in polls.

The super PAC ended 2015 with $58.6 million in the bank.

Anu Narayanswamy contributed to this report.