By Alina Selyukh

WASHINGTON (Reuters) - The bitter media war between Republican presidential candidates Newt Gingrich and Mitt Romney intensified on Thursday when Gingrich's allies released a trailer for a documentary that paints the former Massachusetts governor as unreliable.

"Blood Money" is the second movie to attack Romney this year that is sponsored by Winning Our Future, a "Super PAC" funding group that backs Gingrich.

The minute-long trailer describes Romney as a "progressive" and a "questionable conservative" with a "questionable business background," and flashes up a magazine headline about Medicare fraud.

"This documentary will further expose Mitt Romney's past," Gregg Phillips, Winning Our Future's managing director, told Reuters.

A previous documentary called "King of Bain" from the same group attacked Romney as a greedy destroyer of jobs during his time as the head of Bain Capital, a private equity firm.

Winning Our Future came under heavy fire for that 27-minute documentary, which angered some senior Republican leaders who said it was bashing business.

Since then, the pro-Romney Super PAC Restore Our Future spent more than $10 million on advertising in Florida, which holds its primary on Tuesday. Of that amount, $9.7 million went toward attacking Gingrich, according to federal campaign finance disclosures.

Phillips said the "carpet-bombing" of anti-Gingrich messages in Florida funded by the rival group is what spurred the decision to run a new documentary.

"They did it to us in Iowa and they're doing it to us in Florida, and it's moving numbers," he said. "We have to respond."

Phillips said the group expects the video to go national and plans to buy air time for it. Romney and Gingrich are running about even in opinion polls in Florida.

Watch the trailer here: http://youtu.be/dRz7LrQfIK4

(Additional reporting by Alexander Cohen; Editing by Xavier Briand)