By Gram Slattery and Tim Reid

The head of a political action committee (PAC) supporting Republican Ron DeSantis resigned on Wednesday, dealing a further blow to the Florida governor's struggling White House campaign.

Chris Jankowski, the chief executive of Never Back Down, the main super PAC backing DeSantis's bid to become the Republican presidential nominee, quit after he said in a statement provided to Reuters that his position had become "untenable."

Jankowski's resignation follows departures from within the orbit of DeSantis' embattled presidential bid this year, including the firing of about two dozen campaign staffers in July and the replacement of his campaign manager in August.

Although super PACs are not legally allowed to coordinate with campaigns, Never Back Down and the DeSantis campaign have pushed those boundaries. Never Back Down has organized events across the country at which DeSantis has appeared and has assumed other functions usually carried out by campaigns.

DeSantis began the year in a strong position to challenge former President Donald Trump for the party's White House nomination, but has faded badly in recent months, according to national opinion polls. He trails Trump by nearly 40 points according to a September Reuters/Ipsos survey of likely Republican primary voters.

Trump has become the runaway frontrunner for the party's nomination in what is shaping up to be a likely rematch with Democratic President Joe Biden in the November 2024 presidential election.

Jankowski's statement hinted at internal divisions within the super PAC over not just strategy, but spending.

"Never Back Down's main goal and sole focus has been to elect Governor Ron DeSantis as President. Given the current environment it has become untenable for me to deliver on the shared goal and that goes well beyond a difference of strategic opinion."

Super PACs are allowed to raise unlimited amounts of money to support a political candidate, but are prohibited from contributing funds or coordinating directly with the candidate or their campaign.

DeSantis is banking on a strong showing in Iowa, the state which kicks off the Republican nominating process on Jan. 15. He was given a boost this month when Iowa's popular Republican governor, Kim Reynolds, endorsed him over Trump.

(Reporting by Tim Reid; Editing by Josie Kao)