Now, some of them are vying for jobs in his administration.
The extraordinary repudiation -- partly based on Trump's rejection of basic US foreign policy tenets, including support for close allies -- helped spark the hashtag #NeverTrump. Now, a source familiar with transition planning says that hard wall of resistance is crumbling fast.
There are "boxes" of applications, the source said. "There are many more than people realize."
Some of those applications are coming from the #NeverTrump crowd, the source said, and include former national security officials who signed one or more of the letters opposing Trump. "Mea culpas" are being considered -- and in some cases being granted, the source said -- for people who did not go a step further in attacking Trump personally.
"These are rock solid patriots who have committed themselves" to the security of the United States, but "made a mistake" in signing the letters, perhaps not considering the possibility that Trump could actually win and be in a position to offer them a job, the source said.
In the immediate aftermath of Trump's victory, there has been some concern that the team he would have most difficulty drafting was the one core to US safety and security.
Fifty GOP national security experts signed an August letter saying Trump "would put at risk our country's national security and well-being" because he "lacks the character, values and experience" to occupy the Oval Office, making him "the most reckless president in American history."
Another bipartisan letter cited concern about potential foreign conflicts of interest Trump might encounter as president, and called on him to disclose them by releasing his tax returns. Trump has refused to do so, saying he is under audit and will make the returns public only once that is done.
It remains to be seen what kind of team Trump will pull together, how many "NeverTrumpers" will apply for positions and to what degree the President-elect will be willing to accept them.
There's a fight underway within the Trump transition team about whether to consider "never Trumpers" for jobs, one official tells CNN. New Jersey Governor Chris Christie, who is leading the transition team, has been working to persuade Trump and other top officials to consider Republicans who openly opposed his campaign. That has caused some friction with those who see no place for people who didn't support their candidate.
Some longtime observers of Washington's security scene say it could mean that Trump will still have a relatively thin bench to pull from, compared to other incoming presidents. As a result, the teams he pulls together may be not as effective or cohesive as a result, some say.
"He will get people of variable quality," said Elliot Cohen, an advisor to former Secretary of State Condoleezza Rice who now teaches at Johns Hopkins University. "They will fill those jobs, but the question is, who will they fill them with and what kind of administration will they have."
Trump is already at work on choosing Cabinet secretaries. Names reportedly on his shortlist for the Pentagon include Stephen Hadley, the National Security Adviser to President George W. Bush, Senator Jeff Sessions and Ret. Lt. Gen. Michael Flynn, former head of the Defense Intelligence Agency. Flynn, a Trump adviser, is thought to be under consideration to be National Security Adviser or to head an intelligence agency.
Former House Speaker Newt Gingrich is being floated as a possible Trump secretary of state, as is Bush's ambassador to the UN, John Bolton, Rudy Giuliani, the former mayor of New York, and Senator Bob Corker, chair of the foreign relations committee. And at the Department of Homeland Security Trump is reportedly considering New Jersey Gov. Chris Christie.
But Trump might have more difficulty finding experienced staff just below that level -- undersecretaries, assistant secretaries and deputy assistant secretaries -- for the State and Defense departments.
Cohen, who helped draft one of the letters criticizing Trump, says that people entering his administration should expect it will likely "be torn by infighting and bureaucratic skullduggery." He elaborates that "some of the people will join on to the administration with serious reservations with the guy at the top. And then there will be loyalists. You set up some quite serious tension that is going to be built in."
Others were more sanguine. Howard Schweitzer, a GOP strategist and managing partner at Cozen O'Connor, a Washington law firm, said Trump is a pragmatist and smart enough to know you can't govern the way you campaigned.
"You have to hope he's going to take a step back and realize the enormity of what he has to deal with and surround himself with people who will give him good advice," Schweitzer said. Beyond ensuring a well-run administration, doing so would send an important message to the world that there will be stable, competent continuity, he said.
"He has to show our allies that he's not Doctor Strangelove," he said, referring to the unhinged film character who launches a nuclear bomb.
Still, Schweitzer is among those who says Trump's staffing choices will likely be limited and won't include the Republican party's most experienced security hands. The 50 officials who wrote about Trump in August, including former Homeland Security Secretary Michael Chertoff and ex-CIA and NSA director Michael Hayden, aren't likely to volunteer for the new administration.
The second, bipartisan group of national security experts who warned in September that Trump's overseas business could pose "significant conflicts of interest" included GOP security luminaries such as former national security adviser Robert Blackwill, former Deputy Director of National Intelligence for Analysis Thomas Fingar and former acting CIA Director John E. McLaughlin.
Trump has said his children will run his business while he's in the White House, but experts say that won't be enough to minimize potential conflicts of interest or protect him from potential foreign pressure.
And Omarosa Manigault, the Trump campaign's director of African-American outreach, said there's an enemies list as well. "Let me just tell you, Mr. Trump has a long memory and we're keeping a list," she said.
In the end. Trump may also not prize the kind of expertise and experience represented by the letters writers. In a statement responding to his critics' August letter, Trump said "the names on this letter are the ones the American people should look to for answers on why the world is a mess."
"They are nothing more than the failed Washington elite looking to hold onto their power, and it's time they are held accountable for their actions," Trump said.