Don't let the name of this post fool you; there is no reason to ever consider the adoption of Popular Front tactics.
But let's back up for a moment.
The rise of fascism in Europe caught everyone off guard. From the time the Italian fascists became organized under Mussolini to the time they assumed power in Italy was about two years. At no point was there any wide-spread effective resistance to the fascist onslaught.
The various Communist and Social Democracy sects were horrified by the obliteration of all Left (as well as most Right groups also, for the fascists are not content with half-measures, but now I'm getting distracted) and working class organizations in Italy in so short a time and in such a bloody manner. In response, four general strategies were eventually put forward as a way of fighting back.
The concept of 'fighting', of course, being subjective.
Strategy: Quick, to the constitution!
Early (pre-Second World War) Twentieth Century marxist political parties can be divided into the categories of "reformist" and "revolutionary." When confronted with the Stormtroopers the reformist parties of Germany and elsewhere retreated into the realm of government bureaucracy and constitutional handwringing.
For the Social Democrats of Germany this took the form of the so-called Iron Front. The line of thinking was a simple as it was naive.
"The constitution guarantees a wide range of political and social freedoms. We have the massive strength and resources of the working class as our base of support and the power of various police forces and parliamentary positions at our disposal. Therefore, all we need to do is wait for this fascist 'fad' to pass!"
Needless to say, this failed. Turns out that Hitler wasn't particularly fond of the Constitution.
Strategy: Fascists? What fascists?
Want to see a magic trick?
"The fascists are the party of bourgeois counter-revolution. The Social Democrats are (de facto) the parliamentary screen for bourgeois negation of working class discontent. Therefore, the fascists and the Social Democrats are the same thing!"
Thus the theory of "Social Fascism" was born. The abominable child of the Stalinist clique in the USSR (which had taken power and purged all internal dissent at this point), Social Fascism was their first attempt to grapple with the lessons learned from the defeat of the communists in Italy. Needless to say, they learned all the wrong lessons.
Operating under the theory of Social Fascism the official Communist party in Germany proceeded to act like the party of total lunatics; unsurprising given who was leading them at this point. By holding that the Social Democrats were synonymous with the fascists the Communists completely isolated themselves from the overwhelming majority of german workers who were united under the Social Democratic banner!
In the end the Communists were one of the few groups who bravely took the fighting to the streets against the fascists in 1933, and many of the rank and file gave their lives in the struggle. If only that energy hadn't been so thoroughly misspent by the assholes in Moscow.
For more info: http://wearemany.org/a/2010/06/fascism
Strategy: This does not have to happen.
Which brings us to the United Front.
Trotsky, as well as other leaders of the anti-Stalinist "Bolshevik-Leninist Left Opposition" in exile, put forward the doctrine of the United Front as their solution to the fascist problem. Recognizing that although the Social Demorcats ("marxists in name only") and Nazis were both parties that represented the bourgeoisie, they also contained fatally irreconcilable contradictions between the two of them which put them violently at odds. This in addition to the fact that the Social Democrats had the working class locked up behind them opened up an amazing opportunity for the Communists and other revolutionaries.
Trotsky argued that by publicly calling for full cooperation with the Social Democrats on specific pragmatic goals in the fight against fascism the Communists could reveal the hypocrisy of the Social Democratic leadership, awaken the working classes to action, and obliterate the fascists by drawing the workers into revolutionary struggle.
In order for this to work all members of the United Front would have to maintain full political and organizational independence, except obviously on the specific points where they agreed to cooperate, and retain the right to criticize their allies.
This doctrine was rooted in the fact that the workers of Germany were overwhelming opposed to the fascists, but were held inert and discontented because the Social Democrats feared awakening the working class to direct action, because it would lead to revolutionary fever and threaten their privileged positions within the various bourgeois organizations (parliament, trade union officialdom, etc), and instead redirected the energy of the masses into elections.
By publicly agitating for sensible direct actions against the fascists the Communists would have had the potential to either build a mass movement that would have forced the Social Democratic leadership to engage directly in the life and death struggle against the fascists, or to tear the working class out from under the Social Democrats and into the arms of true revolutionary leadership.
At this point in time Trotsky still seemed to naively trust that the Communists had good intentions despite Stalinist malpractice; within a decade Stalin would have Trotsky murdered anyways, Trotsky who had founded the Red Army and fought alongside Lenin in the 1917 revolution.
Nonetheless the United Front was the only sensible option on the table, and under the leadership of the Left Opposition rather than the Stalinists the Communist Party may have had a strong chance of winning in Germany.
Strategy: Fascists? OH FUCK FASCISTS!
Never somebody to allow an excellent idea go un-ruined Stalin eventually adopted a form of the United Front, following the victory of the Nazis, and rebranded it as the Popular Front. Instead of limiting cooperation to specific points and retaining independence and the right to criticize, Stalin's formulation demanded total subservience of foreign Communist Parties to anyone who would take up arms against the fascists.
The disastrous results of this are too numerous to recount here.
Bringing it home: What does this have to do with the Democrats?
Leading up the mid-1930s the Communist Party in the United States of America had done a lot of excellent work. They led the fight against racism in the worker's movement and were one of the only organizations that never compromised on refusing to segregate any part of their organization, and many Blacks became leaders in the highest positions of power within the party. They were staunch opponents to both the Republican and Democrat parties and refused to compromise or back down on issues of austerity measures or the rights of workers, women (for the most part), and minorities. They fought against the corrupted union bureaucracy and served as a catalyst for many of the most important strikes and factory sit-ins of the time period.
Then the Popular Front ruined everything. Because the Stalinists needed the Roosevelt White House on its side in the fight against fascism the US branch of the Communist party was ordered to throw its full support behind the Democrats. Overnight the Communists disowned all of their accomplishments and towed the Democratic party line on every issue without complaint or reservation.
This was particularly horrendous in the fight for Black equality, which suffered a fatal setback for decades with the de-radicalization of an entire generation of disillusioned Black revolutionaries who were betrayed by the Communists. (Well, I'm oversimplifying a bit, there was a lot that happened to the Black liberation movement during that time, but you get the general idea.)
None of this helped the Communists following the war when the Truman administration led the purges of the Communists from American public life; all of their former allies turned against them because of their betrayals during the Popular Front phase.
But this is the general story of the Left in the United States. The Democrats thrive on cannibalizing leftist organizations. First the Populists, then Communists, now the equal rights and environmentalist and Union activists are all sucked into the Dems' camp by simply pointing a step to the right and screaming "look they're scary!" In return for their unwavering support the Dems give them: nothing. The Dems are the party of seemingly perpetual half-heartedness and compromise. Because every election is the choice between evils, every vote against the Dems is "a vote for the Republicans," the Dems are free to court Wall Street and perpetuate Endless War and cave in to the creeps in the socially conservative lobbies.
The American Democrats are "the second most enthusiastic capitalist party in history" and wear that badge with pride. They support union rights as long as the unions only want the right to campaign for the Democrats. They support women's rights when the voting demographics show that it's a winning campaign issue, and only to the extent that they are forced to. The Democrats whine about Neocon insanity under a Republican president and then entrench and codify the same "shredding of the constitution" and "senseless imperialism" as soon as they are elected.
The Republicans aren't fascists, but they are a party of extreme right-wing insanity. The American Democrats aren't the German Social Democrats, but they are close enough to serve our purposes.
The argument goes that we need to support the Democrat leadership, that we can pull them to the left over time. This is patently just drivel. If the Democrats truly want support against the right-wing that's nice, I'll support that. But we on the Left cannot continue to surrender our independence, our energy and organizational integrity to the Democrats every time they scream "It's an election and we're the lesser of evils!"
If they want our support they will have to take action that we can support. They must take seriously universal healthcare, worker's rights, electoral reform and true universal suffrage, equality regardless of gender, creed, ethnicity or nationality, end the war on drugs, the war on Muslims, and a million other things they pay lip service to.
Reverse the usual agitprop and look at it this way: if the Democratic leadership wants to win the fight against the Republican insanity they are going to need us. We, the workers, the poor, the students, everyone who wants a better world, must make it clear that the Democrats will need to join our United Front.