Borah would lose almost as badly as Landon, but I wonder if there would even be a Lemke third party candidacy? Father Coughlin and Borah got along well; in particular they cooperated in defeating US membership in the World Court. And Borah in OTL actually spoke at a Lemke campaign rally in October 1936:
https://www.nytimes.com/1936/10/12/archives/borah-at-lemke-rally-speaking-in-place-of-gerald-smith-he-praises.html
I do think it would be a little bit harder to portray Borah as a tool of the "economic royalists" than Landon. Yes, Landon had a progressive background--he had been a Bull Mooser in 1912--but in 1936 a lot of his support came from conservatives who tried to portray him as a "Kansas Coolidge" who had succeeded in balancing the Kansas budget. (The "Kansas Coolidge" characterization was disliked by Landon himself and was undoubtedly misleading, as Arthur Schlesinger, Jr. has acknowledged.
https://books.google.com/books?id=88OMakRtI0EC&pg=PT548) Borah tried to contrast himself with other Republicans by denouncing "'The high place in the counsels of the party which corporate and monopolistic interests have long occupied,'...(mentioning oil in particular — a presumed slap at Landon)."
https://books.google.com/books?id=vC5HJloBWugC&pg=PA540
Allan J. Lichtman's summary of Borah's campaign is interesting:
"Front-runner Landon was the lone Republican governor to have won re-election in 1934. He had roots in the party's progressive past, but the right found him sufficiently pliable for their purposes. His lack of national experience and stature might have mattered in a year of bright party prospects, but not in 1936. With Vandenberg looking to 1940 and Knox fading, the seventy-year-old progressive senator William Borah of Idaho emerged as Landon's main rival.
"Borah's backers included progressives mingled with a few conservatives, led by newspaper magnate Frank Gannett. The publisher touted Borah as the only Republican with a chance to beat President Roosevelt. For Gannett, any Republican was preferable to FDR, whose policies "might be called Fascism, Nazism, or Communism. It all amounts to the same thing." The president's "castigation of the Liberty League and the big interests,. Gannett told Borah, "makes it pretty difficult for the Republicans to consider a candidate representative of that class... You are the only candidate in the field who is immune to this attack." Borah's backers argued that "the main issue in the next Presidential Campaign will undoubtedly revolve around the Supreme Court." They warned that if the Court were "packed by the present Executive ... socialism would be wrapped around our neck for 25 years." The conservatives behind Borah admired his social and cultural views and believed his opposition to concentrated power would incline him to appoint Supreme Court justices skeptical of bureaucratic expansion. But Borah refused to repudiate his support for tariff reduction, generous aid to the needy, the dissolution of corporate monopolies, and other progressive policies that alienated most of the GOP's economic conservatives. in a letter to Frank Gannett, he wrote, "The price [demanded by economic conservatives] is entirely too high for their support or even the presidency.... I am not buying the Presidency, Gannett, by surrendering my convictions upon public questions." Borah kept his integrity and Landon won the presidential nomination."
https://books.google.com/books?id=3q92ePfQDloC&pg=PA87
On the Supreme Court issue, one should note that Borah had joined Norris and La Follette in opposing Hoover's nomination of Hughes to the Court, arguing that Hughes' confirmation would result in "great economic oppression to the people of the United States."
https://books.google.com/books?id=sPBnA3fpPI4C&pg=PA179
All in all, I would say Borah would do marginally better than Landon. But FDR would still easily win. (He might even do a little bit better with the African American vote than he did in OTL, Borah being notorious as an ally of the South on racial matters.)