The candidate O’Donnell has worked with most closely over the past decade has been Arkansas Sen. Tom Cotton. Since Cotton’s first run for Senate in 2014, O’Donnell has helped the senator with debate prep, media training, strategy and messaging. “The senator and Mrs. Cotton value Brett a lot,” said Cotton’s longtime communications director Caroline Tabler.

Tabler said that O’Donnell’s debate prep advice to Cotton was often to simplify a message, hone a line of attack and return to it consistently even if it felt repetitive to drive home the point. During his 2014 debate against the incumbent Democratic Sen. Mark Pryor, Cotton took flak for tying Pryor to Obama over and over again on the debate stage — but that message helped bury Pryor in the heavily Republican state and put Cotton in the Senate.

During this time, O’Donnell got caught up in campaign finance troubles. As POLITICO has previously reported, the Office of Congressional Ethics investigated whether then-Rep. Paul Broun (R-Ga.)’s office had paid O’Donnell for campaign work using official office funds — aka taxpayer money — which are possible violations of federal law as well as House rules.

O’Donnell pled guilty to a false statement charge related to the investigation in 2015, admitting to “knowingly and intentionally” making several false statements to the Office of Congressional Ethics in an effort to minimize “and conceal the true nature and scope of” his work for Broun’s campaign. He was sentenced to two years’ probation and a $10,000 fine in 2018. According to court documents, O’Donnell testified that Broun’s staff had pulled a “bait and switch” on him by adding campaign work onto his official job duties.

O’Donnell has also done work outside of U.S. politics: He advised the campaign that pushed Brexit and later served as an adviser for former British Prime Minister Boris Johnson’s 2019 reelection.

But his most regular campaign work has come from a decade-long relationship with the National Republican Senatorial Committee, which has used him as a debate coach on multiple races and for general messaging advice. In April 2020, as criticism mounted against Trump’s Covid-19 response, O’Donnell penned a 57-page memo for the NRSC, first obtained by POLITICO, that advised GOP candidates to largely sidestep the president and pivot to blaming China for the pandemic. “Don’t defend Trump, other than the China Travel Ban — attack China,” he advised in the memo.

That message infuriated Trump’s campaign, which privately threatened to cut off any Senate candidate who followed O’Donnell’s advice. As POLITICO reported, NRSC officials quickly backtracked and made clear to Trump advisers they weren’t advising candidates not to defend Trump. The memo also irked China’s leaders: China Daily, a propaganda outlet published by the Chinese Communist Party, published an editorial attacking O’Donnell and his GOP allies.

O’Donnell has also worked for another of the GOP’s most vocal China hawks. He has served as an adviser to former Secretary of State Mike Pompeo since at least 2015, when Pompeo was still in the House, and he advised Pompeo on message and strategy before Pompeo decided against a presidential bid.

One GOP strategist who’s worked with O’Donnell on multiple races but asked for anonymity because he is currently supporting another presidential candidate called O’Donnell a “pro” who prepares his candidates well for debates but said that he has “a tendency to push candidates hard right.”

While O’Donnell is busy helping DeSantis ahead of his first presidential debate, there’s only so much he — or any campaign adviser — can do. Debates are the most unpredictable, least stage-manageable parts of a campaign, as the candidate is stuck on stage alone under the spotlights.

Advisers can’t remake a candidate — and even a strong debate performance can only accomplish so much. “DeSantis is over. Put a fork in him. No debate coach can change that. The problem is Ron,” former Republican Rep. Barbara Comstock, a fierce Trump critic who wants to see him denied the nomination, said when asked if O’Donnell could help DeSantis turn things around.

DeSantis has not proven himself to be a rockstar debater in the past. His 2018 gubernatorial debate against Democrat Andrew Gillum devolved into a volley of character attacks and yelled insults that left both looking churlish. Four years later, DeSantis’ refusal to offer any answer on whether he’d serve a full term — including a few seconds of awkward silence after Crist prodded him — drew negative headlines.

Stevens, the former Romney adviser who briefly worked with O’Donnell on Romney’s 2012 race and has since quit the GOP in protest over the direction the party has headed during the Trump years, also worked for Crist’s early campaigns before the lawmaker left the GOP in 2010. He said “DeSantis froze like a deer in the headlights,” and “lost a debate to Charlie Crist, which is not particularly easy to do.”

“It’s smart of him to realize he needs more help,” Stevens said of DeSantis. “I’m sure Brett will do a good job for him.”

But how much that can accomplish remains to be seen.

“The one thing you can never do in debate prep or in any candidacy is remind your candidate of their deeply held beliefs. You might as well take a guy to a dance and tell them which person they’re going to fall in love with and marry,” he continued. “The problem is humpty dumpty, not the guys trying to put him back together.”