What decade does it become too risky to use your time machine to kill Hitler?

The Weimar system of multi-party parliamentary democracy and the crazy fragmentation of the German electorate made this really unlikely. (Hitler — or rather Hitler’s party — never won a majority nationally. And since all the non-Nazi parties hated the Nazis, he probably would never have won a parliamentary majority.)

Sensible party leaders who were ready to form coalitions would have been more plausible than a single party that could command majority support. I think.