Below is the wiki biography of Wendell Willkie, who did not run in a single primary and had only recently converted from Democrat to Republican, yet stole the 1940 Republican nomination away from Taft, Vandeburg and Dewey at the convention:
https://en.wikipedia.org/wiki/Wendell_Willkie
"The 1940 Republican National Convention opened at the Philadelphia Civic Center[a] on June 24, 1940. As the delegates assembled, they discussed the war, the candidates, and Roosevelt's appointment of two Republican interventionists to his cabinet four days before the convention. Henry L. Stimson, Secretary of War under President Taft and of State under Hoover, was restored to the War position, and Landon's 1936 running mate, Frank Knox, was appointed Navy Secretary. The cabinet appointments divided the Republicans, who accused Roosevelt of dirty politics.
Willkie arrived by train in Philadelphia on June 22, two days before the convention, and immediately attracted attention by walking from 30th Street Station to his hotel, answering questions from reporters and anyone else who could get close enough to be heard. Dewey, Vandenberg and Taft had large public headquarters, but Willkie's campaign was run from clandestine rooms at the Benjamin Franklin Hotel. Root's Willkie Clubs and other supporters bombarded the delegates with telegrams urging support for their candidate, to the annoyance of some. Key convention officials were Willkie supporters; these included House Minority Leader Joe Martin, Massachusetts' favorite son and permanent chairman of the convention. When the head of the Committee on Arrangements, Ralph Williams (deemed likely to support Taft) died just before the convention, he was succeeded by the vice chairman, Sam Pryor, a firm Willkie backer. This placed a Willkie supporter in charge of tickets for the public galleries.
The opening night of the convention saw the keynote speech by Governor Stassen; he subsequently announced his support for Willkie and became one of the candidate's floor managers. The second night featured a speech by the only living former president, Herbert Hoover, who hoped to stampede the convention to a third nomination. His address went almost unheard in the hall because of problems with the sound system. In the meantime, the Dewey campaign, faced with the German announcement that with France taken, Hitler's forces would sail on Britain, did its best to stem the flow of delegates to Willkie. Negotiations among Dewey, Taft, and Vandenberg came to nothing as none was willing to accept less than the presidential nomination. A blaze of publicity followed Willkie wherever he went, as he caucused with delegates and appeared at press conferences with supporters, including the entire Connecticut delegation. A strong minority of African Americans still supported the Republicans, and Willkie met with a group of them, urging those delegates to visit him in the White House in 1941.
Indiana Congressman Charles Halleck gave the nominating speech for Willkie on the evening of June 26, arguing that Willkie's recent conversion to the Republican Party was no reason not to nominate him, "is the Republican Party a closed corporation? Do you have to be born into it?" When Halleck mentioned Willkie's name, there were initially boos from some delegates, but they were quickly drowned out by those in the public balconies, who thunderously chanted, "We want Willkie!". Pryor had cut ticket allocations to delegations that were not for Willkie, and distributed thousands of standing room passes to Willkie partisans. The vocal support for Willkie among spectators led to complaints that other campaigns had been shorted in the distribution of tickets, but provided one of the convention's most dramatic moments.
Dewey had predicted he would have 400 of the 501 votes needed to nominate on the first ballot and kept nothing in reserve so that he might show momentum in future ballots. When delegates first balloted on the afternoon of June 27, he had only 360 to 189 for Taft, 105 for Willkie, and 76 for Vandenberg. On the second ballot, Dewey began to slip, falling to 338 to Taft's 203 and 171 for Willkie. The losses greatly damaged Dewey's campaign, as other than the trivial losses suffered in the early rounds of balloting by Warren G. Harding in 1920, no Republican candidate had ever lost support from the previous ballot and won the nomination. Dewey came under pressure from his advisors to withdraw during the dinner break that followed the second ballot, and when the convention resumed to chants of "We want Willkie!" from the packed galleries, Dewey continued to slip as the convention became a two-horse race between Taft and Willkie. Listening by radio from his hotel room, Willkie refused to make a deal to get support from Taft delegates in exchange for making the Ohioan his running mate, and became convinced he would lose on the fifth ballot. Dewey had planned to go to the convention and withdraw, hoping to stop Willkie by endorsing Taft, but by the time he decided this, the fifth ballot was about to begin and he could not get to the Civic Center in time. Willkie led with 429 delegates after the fifth ballot, while Taft held 377 and Dewey only 57. The large states whose votes still were not committed to one of the two leaders were Pennsylvania (Governor Arthur James was the favorite son) and Michigan, most of whose delegates stayed with Senator Vandenberg. Although Willkie had refrained from making deals to this point, to get Michigan, he agreed to allow the Republican organization there to pick that state's federal judges. The sixth ballot, held at 12:20 am on June 28, saw Taft, then Willkie take the lead. As those in the gallery continued to call for Willkie, Vandenberg released his delegates, most of whom went to Willkie. Pennsylvania also broke for him, making Willkie the Republican nominee for president on a vote that was made unanimous.
Willkie had offered the vice presidential nomination to Connecticut Governor Raymond Baldwin, a key supporter, but scuttled those plans after his advisors and Republican officials felt that a New York-Connecticut ticket would not give sufficient geographic balance. They urged Willkie to select the Senate Minority Leader, Oregon's Charles McNary. A lawyer, advocate of public power, and farmer, McNary was popular and respected in the West. Willkie agreed, and got Baldwin to withdraw as others persuaded McNary, who had called Willkie a tool of Wall Street after arriving in Philadelphia. The convention dutifully nominated McNary. Before departing Philadelphia, Willkie went to the Civic Center to appear before the delegates who had chosen him, becoming the first Republican nominee to speak to the convention after gaining its endorsement:
"Democracy and our way of life is facing the most crucial test it has ever faced in all its long history; and we here are not Republicans, alone, but Americans, to dedicate ourselves to the democratic way of life in the United States because here stands the last firm, untouched foothold of freedom in all the world."