It’s generally not a good idea to compare American politics with British politics because the center left in America is not nearly as far to the left as the British center today, but especially 30 or 40 years ago when the Labour Party still had very strong Marxist elements. You would have better luck finding Evangelicals who embrace pornography, homosexuality, and prostitution in America than you would be able to find almost anyone who considers himself a Marxist.
America is a Federal Constitutional Republic in the form of a liberal democracy and Britain is a unitary monarchy in the form of a social democracy with no national constitution. So British politics, Left or Right, is much further left than American politics, at least when it comes to the two centers, but the American far left doesn’t look much different ideologically from the center left in Britain. The two centers in America are Liberals on the Left and conservative libertarians on the Right. The two centers in Britain are socialists on the Left and conservative social democrats on the Right.
But what both countries have in common is that the two major parties in both countries have broad coalitions and diverse ideological factions in them because neither country has two dominant political factions big enough to occupy their party on their own yet govern the country by themselves. So you see the radicals on both sides of the pond trying to get those they want in top leadership positions and their policies addressed, and to push this as hard as they can but often ending up settling for the best of what whoever is in charge gives them.
And because of this I like to look at the Democratic Party as a party of JFK/Bill Clinton Liberals and FDR/LBJ Progressives who are generally in charge of the party, at least since post-1988 but even post-1984, that moved past the McGovern coalition to become a mainstream center left party. But there is still that George McGovern social democratic coalition that is anti-war, anti-military, anti-law enforcement, anti-corporate, and even anti-profit in a lot of cases, and wants to make America into a British or Scandinavian unitary government in which most of the governmental power would rest with the Federal state.
And because of this, and with the growth of both Liberals and Progressives in the Democratic Party, younger voters tend to be very liberal on social issues but also tend to be business owners or managers of small businesses or perhaps even military veterans from the Afghanistan or Iraq war. They tend to be liberal on social issues but not anti-corporate, anti-business, or anti-military, and of course I’m not saying Liberals are, because we aren’t, but young people in America tend not to be social democrats on the far left who believe they are under-taxed and that people have too much individual power.
Because of this, unlike the Republican Party, the Democratic Party doesn’t need its fringe to be successful. Without the Democratic Party, the far left wouldn’t be heard much in American politics. I mean, who the hell is Dennis Kucinich without the Democratic Party? Most of the country would have never heard of him and because of this going forward, the far left would be better off outside the Democratic Party, attempting to create a united national socialist or social democratic party they could call home.