Republican presidential candidate Ben Carson's campaign manager and communications director on Thursday resigned from their posts, causing further strife within the campaign that has suffered a slump since it hit its peak in November. Carson's campaign manager Barry Bennett and communications director Doug Watts gave Carson their notice, according to the Des Moines Register.

In a statement to the Des Moines Register, Watts said: "Yes, Barry Bennett and I have resigned from the Carson campaign effective immediately. We respect the candidate and we have enjoyed helping him go from far back in the field to top tier status. Having just announced raising $23 million for the fourth quarter, more than any other Republican candidate, and passing one million contributions and and over 600 million unique donors. Since March, we are proud of our efforts for Dr. Carson and we wish him and his campaign the best of luck."

A shake-up was expected to take place next week in the Carson camp, in which Carson said "every job is on the table." He added: "We're going to certainly be altering some things in the campaign, and we will be talking about that quite publicly next week," Carson said on Fox Business, HNGN.com previously reported.

The changes come just a month before the Iowa caucus, in which the retired neurosurgeon was once expected to do well when he had 28 percent support in the state in October. He has since plunged 15 percentage points, according to an early December poll by Bloomberg Politics.

Carson's campaign has raised more than $23 million in the fourth quarter of 2015, Politico reported on Wednesday. However, it would not say how much of that cash it has on hand.

Carson has shown a steady decline in the polls since peaking last month. RealClear Politics shows Carson polling at a 9.4 percent average after nearly reaching 25 percent in early November.