"Stop tweeting" is tops on the list of things Trump should avoid, Jenkins said in the wake of spiraling negative headlines surrounding the real estate mogul's feud with the parents of a U.S. solider who died in Iraq and his refusal to back House Speaker Paul Ryan's re-election.
In an interview on CNBC's "Squawk Box," Jenkins said Thursday he would also tell Trump: "We're going to start doing some TV commercials that paint you as a serious businessman who gets things done. [And] your rallies are not going to be so free form."
Trump's missteps are playing into the hands of Democratic nominee Hillary Clinton, the Journal editorial board member said. "[Clinton's] whole campaign shtick now is: 'I'm not Donald Trump. You cannot take a risk on this crazy person. I'm your only choice.'"
Clinton may be right, said Jenkins, whose writings have been rather critical of Trump.
"The optimal outcome for the country, I'm guessing, maybe [is] a narrow Hillary victory with Republicans strongly in control of Congress; both parties having received a scare from Donald Trump and getting serious about some pro-growth reform," Jenkins said.
Jenkins added he does not want Trump to perform so poorly in the election that voters abandon Republican congressional candidates. "I don't want to see a wholesale flight from Donald Trump."
"I was rooting for Donald to change, ... to broaden his appeal, to prove he had the judgment and the discipline to be president as well to be a very successful rabble-rouser among a certain very discontented part of the American public," he said.
But post-convention, "I haven't seen the Donald Trump I wanted to see," Jenkins said, echoing criticism the columnist levied in a June CNBC appearance, in which he wondered whether Trump even wanted to be president and called on Trump to tone down his fiery rhetoric.