He wasn’t and opposed to what?
Well, Hermann Müller was the last chancellor of the Weimar Republic that had the support of a majority in parliament and governed with the support of a parliamentary majority.
Brüning as chancellor is a bit of a grey area and, as far as I know, historians are divided how to judge. Brüning only commanded a minority in parliament and signaled that he was willing to govern against the majority of parliament (with support of von Hindenburg) if needed, governed via emergency decrees by von Hindenburg (article 48) and basically pressured/blackmailed the SPD into tolerating his government (sustaining Brüning against votes of no confidence and voting against overturning the emergency decrees, which was within the authority of parliament) by threatening to call new elections if the SPD refused to support him, which would have brought have further gains for the NSDAP (calling new elections because the SPD refused to support his budget was exactly what led to the 1930 elections that saw the first major gains for the NSDAP). While his conduct as chancellor was constitutionally legal, there is ample justification to say that Brüning wasn’t really elected democratically and governed with very questionable democratic legitimacy.
Von Papen was even worse as chancellor he had even less support in parliament then Brüning and when he was about to be brought down as chancellor in a vote of no confidence (the vote was 42:512 against von Papen), he had von Hindenburg hastily dissolve parliament to stay in office and asked von Hindenburg to postpone elections indefinitely and declare a state of emergency (von Papen was an enemy of democracy and from the beginning of his chancellorship hoped to pave the way for a dictatorship), which not even von Hindenburg and right-wing officers of the Reichswehr were willing to support. Von Papen’s conduct as chancellor was constitutionally questionable (his de facto coup in Prussia was declared illegal by German constitutional court but von Papen ignored the judgement) and cannot be called anything but authoritarian.
Von Schleicher, the next chancellor, had no support in parliament whatsoever, failed to gain any, asked von Hindenburg for the same thing von Papen had before him (ironically his opposition to von Papen’s demand to von Hindenburg of postponing elections indefinitely and declaring a state of emergency and declaring that the Reichswehr couldn’t win the resulting civil war convinced von Hindenburg to decide against von Papen) and was also denied by Hindenburg.
Not even Hitler ,when appointed as chancellor in 1933, commanded a majority in parliament, governed with emergency decrees issued by von Hindenburg, and shortly afterwards destroyed democracy for good (Brüning to a degree, and definitely von Papen and von Schleicher, plus von Hindenburg as president had already weakened it signifitcantly).
There is a reason that the cabinets of Brüning, von Papen, von Schleicher and Hitler (until he obtained a majority in parliament in the clearly undemocratic elections of March 5 1933) are classified as Präsidialkabinett (presidential cabinet) or even Präsidialdiktatur (presidential dictatorship) by German historians. They didn’t govern with the support of parliament but by asking von Hindenburg to issue emergency decrees; even Brüning, who pressured the SPD into tolerating his minority government didn’t make laws or budgets with the support of parliament but had von Hindenburg issue emergency decrees and then had his minority coalition, and grudgingly the SPD, vote against overturning the emergency decrees. It is entirely legitimate to think of the period between the fall of the Grand Coalition under Müller in March 1930 and the Reichstag Fire Decree in February 1933 as a transitional period, where Germany was no longer a full democracy but not yet a dictatorship (with the right PODs and a lot of luck the process could have been reversed, but history played out otherwise).