"In his posthumously published memoirs Brüning claims, without support of contemporaneous documents, that he hit upon a last-ditch solution to prevent Hitler from taking power—restoring the Hohenzollern monarchy. He planned to persuade the Reichstag to cancel the 1932 presidential election and extend Hindenburg's term. He would have then had parliament proclaim a monarchy, with Hindenburg as regent. Upon Hindenburg's death, one of Crown Prince William's sons would have been invited to assume the throne. The restored monarchy would have been a British-style constitutional monarchy in which real power would have rested with the legislature.[citation needed]
"He managed to garner support from all of the major parties except the Nationalists, Communists and Nazis, making it very likely that the plan would get the two-thirds majority required for passage. The plan foundered, however, when Hindenburg, an old-line monarchist, refused to support restoration of the monarchy unless Emperor William II was called back from exile in the Netherlands. When Brüning tried to impress upon him that neither the Social Democrats nor the international community would accept any return of the deposed emperor, Hindenburg threw him out of his office.[5]..."
https://en.wikipedia.org/wiki/Heinrich_Brüning
Let's assume that Brüning actually did plan this--and that Hindenburg had gone along. Could the plan have gotten a two-thirds vote in the Reichstag elected in 1930? (Remember, this has to be before the March 1932 scheduled presidential election, and therefore before the July 1932 Reichstag election, where the NSDAP and KPD combined got a majority of the seats, obviously making it imposible for Brüning's alleged plan to get a majority, let alone two-thirds.) In the Reichstag elected in 1930 the NSDAP and KPD combined had 31.88% of the seats--not enough to block the measure if all other parties agreed to it. The key therefore is the attitide of the DNVP which had 7.11% of the seats.
https://en.wikipedia.org/wiki/German_federal_election,_1930 On the one hand, it might seem strange that many deputies from the DNVP--which was a monarchist party at heart--would want to oppose restoration of the monarchy. OTOH, it actually shouldn't be so surprising they objected to this plan. After all, the *immediate* effect ot the plan sould be to extend Hindenburg's stay in power, and in 1932 there had been a total reversal of German right-wing attitudes toward Hindenburg compared with 1925. According to one ecological regression estimate, about 50 percent of first-round Duesterberg voters in 1932 supported Hitler in the second round, compared to only 15 percent who supported Hindenburg (about 28 percent didn't vote at all and 7 percent voted for Thalmann). Table 9B in
https://core.ac.uk/download/pdf/42110234.pdf
Still, if Hindenburg had pressed for the measure, probably some DNVP deputies could accept that even a constitutional monarchy headed by someone other than Wilhelm II and supported by Social Democrats would be a lesser evil than a continuation of the republic--or even the dictatorship of a plebian Führer whose wooing of conservatives was pretty clearly opportunistic. So let's say that with the support of some though not all DNVP deputies, Brüning's plan is enacted.
Would this actually prevent an NSDAP government? I doubt it. Very likely Hitler would have forced his way into the Chancellorship (after all he headed the largest party in the Reichstag after July 1932 and many non-Nazi German conservatives were willing to acquiesce in a Hitler-led government) and manipulated the young Kaiser into playing the same figurehead role for him the King of Italy played for Mussolini when the latter established his one-party dictatorship. (A standard joke of mine in replying to people who lament the end of the German monarchy in 1918: "It's a good thing that Italy and Japan remained monarchies after World War I. Otherwise, they might have gone fascist...")