Well firstly we need to prevent Einstein emigrating to the US. So our point of divergence needs to be well before 1932. Indeed we need to make Einstein 'get political' in line with the rise of the NASDAP between 1925 - 1930.
It might be a good idea to say that some how he gets caught up in the events between the SA and Communists in 1928, this effects him enough to become outspoken against the Nazis.
However the Nazis at the time are specifically looking for publicity, and the saying goes; any publicity is good publicity.
Therefore instead of Hitler and his propagandists in the party trying to shout down Einstein who is already a well respected and famous figure, he is instead invited to a 'dinner party' so that he and Hitler can meet and exchange views.
Of course the 'dinner party' also includes other NASDAP members as well as other political figures. Hitler being the shrewd cunning but otherwise not so intelligent man he is, and Einstein having enjoyed a bit of drink the conversation between them goes something like this;
Einstein ridicules Hitler for his 'violence in the streets' and use of common prejudices and overall appears to be 'winning the opinions over' of the rest of the guests, since much that he says makes sense.
Hitler meanwhile goes quite with a form of silent rage at this, finally making an abrupt remark that; "Perhaps Her Einstein should join the Party such that we can stop making these kinds of mistakes!"
The sarcasm is quickly lost on that statement when one of the other guests comments that it might not be such a bad idea. At which point Hitler realises rather than having lost this debate might actually be able to save himself in front of the other important guests.
So he presses the issue to Einstein about if he is 'serious for politics' then maybe he should join him [Hitler].
Einstein is now a bit on the backfoot, since if Hitler is asking him for advice and sense, which is what he [Einstein] has been advocating the Nazis need, then he would loose face not to accept. Although the idea abhors him, through a bit of drink, the notion that he might have lost the morale high ground to Hitler in front of the important figures in German society and he might have been given an opportunity to do some good, he accepts the offer.
In the intimidate aftermath of this, the other figures in the party realise that while Hitler is a great orator for the masses, Einstein 'being on board' although fairly remotely, gives them far more credibility in the minds of high German society. This means that for the party, Hitler can only go 'so far' at attacking Jews and intellectuals. Since of course Einstein needs to be kept onboard.
Therefore slowly his speeches turn from degrading the Jews to more specifically the 'Communists in their midsts' [Nod towards the US McCarthy era]
Therefore it becomes the Communists and socialists that becomes Hitler’s scapegoats by the time of 1929 referendum and on towards 1933.
During this time, Einstein has a minor role at being able to curb the violence associated with the Party, particularly at being able to belittle Hitler behind the curtain within the inner ranks of the party. Although it disgusts him about how the party is achieving power, he too is caught up on a feeling of power and spirit. Meanwhile he realises that he is becoming a strong second in the party.
Secretly he hopes that Hitler will be found guilty by one of the new laws against political violence, and thus he can take over and impose his stamp upon the party for the better.
Of course Hitler realises this and so in the 'Night of Long Knives' tries to have Einstein murdered along with those dissenters in the SA. Einstein gets tipped off by others within the party and escapes the purge but not without working out who ordered it.
Rather than confront Hitler directly he flees to Austria to get a sense of what his next move should be and also possibly because he resents his decision to get involved with politics.
Over the next few months as Hitler declares he is the "supreme judge" and Fuhrer of Germany, others in the party are concerned that without Einstein being in the picture, suddenly Hitler has gone power mad and crazy, thus begging Einstein to come back.
Fortunately or unfortunately, Hitler is shot himself in 1935 creating a massive power vacuum. It is at this point that Einstein regretfully takes up the position at the head of the Nazi party since he is still well liked by the public, and appears to be one of the best candidates to take over.
In doing so, he assumes all the "supreme power" that Hitler declared for himself however even with this 'power' he can't do much to turn the tide of popular opinion that is the "Thousand Year 3rd Reich", and the fact that Hitler becomes a Martyr for his ideology....