CONCORD, N.H. — Ben Carson, the famed neurosurgeon whose bid for the Republican presidential nomination has struggled to keep pace with rivals, will cut more than 50 staff positions Thursday as part of an overhaul and downsizing of his campaign.

Salaries are being significantly reduced. Carson’s traveling entourage will shrink to only a handful of advisers. And instead of flying on private jets, Carson may soon return to commercial flights.

The employees being released — about half of Carson’s campaign — mostly work in field operations and at his headquarters in Northern Virginia.

Campaign officials, who confirmed the moves after The Washington Post obtained an internal memo about the layoffs, stressed that key aides in upcoming GOP primary contests will remain in place and that Carson is determined to stay in the 2016 race.

But they acknowledged that Carson’s funds have diminished as he has fallen from the top tier, forcing him to make sweeping changes to a campaign that had swelled into a bustling operation of about 125 people.

Carson’s senior staff, which was restructured in late December after several advisers resigned, will not change, the campaign said. Retired Army Maj. Gen. Robert F. Dees, Republican consultant Ed Brookover, and communications advisers Jason Osborne and Deana Bass will stay, among others.

Longtime Carson confidant Armstrong Williams said the drastic shake-up has been under consideration for weeks but was postponed until after Monday’s Iowa caucuses, where Carson finished in fourth place with 9.3 percent of the vote.

“Dr. Carson is going to get his campaign lean — really lean,” Williams said in an interview. “One issue for a while has been too much infrastructure, and he has decided to fully address it so that he can sustain his campaign until the convention.”

Williams added that Carson remains incensed at Sen. Ted Cruz of Texas, who won the Iowa vote, over the way the Cruz campaign incorrectly told Iowans on Monday night that Carson was quitting the race. Carson has called those efforts “dirty tricks.”

Although Cruz has apologized to Carson and said his staff was reacting to a news report, Williams said Carson is now as determined as ever to fight on and battle with the senator for the support of evangelical Christians and tea party activists.

“I make no bones about the fact that I am a person of faith. I believe what it says in Matthew 7:20, that 'By their fruits, you will know them,'" Carson told reporters Wednesday at an impromptu news conference in Washington. "I hope you will judge all the other people who are running for this position by the same standards."