It seems to me that the UF isn't essentially different from the popular front besides nominal adherence to the workers movement.

After the Comintern's disastrous class-collaborationist policies in China they did an about turn and began denouncing even the social-democrats in europe as "social-fascists". Instead of the previous collaborationism they implemented the policy of a "united front from below".

This was in fact the same line taken by the left wing of the PCd'I (At that time also the leading wing) in response to the Comintern's call to form united fronts:

http://www.leftcom.org/en/articles/2...nalist-history

Of course Trotsky criticised this by calling for a united front of the workers movement, which also put him in something a position to criticise the later tactic of the popular front.

However it seems questionable to me wether the social-democracies nominal orientation towards the worker movement would be enough to make them worthy allies. In essence the social-democratic parties were bourgeois parties which promoted nationalism, statism and class collaborationism (Despite making pseudo-left criticisms of capital), so there doesn't seem to be any essential difference between an alliance with SD's and with bourgeois liberals. In fact I recall Mike Macnair in his recent talk on fascism saying that in point of fact it was also the social-democrats who called for the defence of bourgeois elements against the workers movement as a condition for the popular front.

Do any Trots have a defence of why the UF is essentially different from the PF?

I wouldn't describe myself as a Trotskyist, but I do see some significant differences.

My understanding is that the UF tactic wasn't designed to win over the social-democratic parties, but rather it was a recognition that these parties held the allegiance of huge numbers of workers and workers organisations. Thus, the purpose was on the one hand to defend the institutions of workers power against capitalist attack, and on the other to demonstrate in practice to workers the contradictions of social-democracy and thus bring them into the communist fold. As such, the UF was always accompanied by criticism of the social-democratic leadership.

Due to this, when communists first started trying to operate a UF after the failure of the Third Period, social-democratic parties were extremely wary. If I recall correctly the French socialist party (where the UF/PF started on a mass scale) refused to enter into a UF until the communists agreed not to criticise them, due to their observations of UF tactics pre-1928.

The PF differed in that it was no longer about winning over and strengthening the working class, but rather it was about opposing fascism at any cost. Thus communists attacked strikes and occupations, and supported bourgeois governments. In practice they often moved further to the right than the social-democrats they had been so fiercely attacking just a couple of years earlier, necessarily (as a result of allying with bourgeois parties) taking a class collaborationist stance. This is demonstrated by the middle class/student/intelligentsia following that the communist parties gathered during this time.

That's not what I gathered. My understanding was that it was assumed that the working classes would be won over to the Communist party when it was seen that the communist's were the best fighters for the aims of the United Front.

Also, bourgeois-democratic parties have often had significant hold over the working class. The earliest labour candidates in england were simultaneously labour candidates. The labour movement in america is still tied for the largest part to the Democratic party from what I've seen.

However, the communists made very few criticisms of the other parties (except when it came to Spain) and instead followed a line of unity and nationalism. This is clearly different to the way the UF was meant to work.

The bourgeois parties pick up working class votes as well, of course, but if socialism is your aim then there is nothing to unite with these parties on. There are thus far better (and easier) ways to expose that these parties are opposed to working class interests.

The social-democratic parties in the 1930s were, of course, a million miles to the left of the mainstream parties we have in the west today. That said, one might argue that there are still reasons to form United Fronts with today's parties, whether to defend an industry from privatisation, support pro-union legislation, or anything else of that sort.

If socialism is your aim then there is nothing to unite with the social-democratic parties either though. They held back the working class in revolutionary periods in Germany and Italy and there is no reason to suppose that they would not do so again (And quite a number of reasons to suppose that they would), which seems to make it almost suicidal to give any support to them (Besides the fact that currently the danger from fascist attacks on the workers movement is much lower than it was when the united front was originally devised).

However, the social-democratic parties also had an interest in defending working class institutions and many of the gains. This was the major point that Trotsky was making against the idea that the social-democrats were the greater enemy -- if the Nazis got into power they would destroy working class parties and trade unions, while the SPD had a strong interest in defending them.

Obviously on the eve of revolution you aren't going to go and ask for the SPD's permission, but that is missing the point. It is the difference between the tactics used to defeat Kornilov, when you call out all of your potential forces to defeat the common enemy, and the tactics used to defeat Kerensky in October, when many of those forces would hold you back.