I define "the left", broadly speaking, as spanning the spectrum from left liberals through to anarchists - the type of people who would march together for democratic rights, even though divided on the principle of whether to reform capitalism or overthrow it.
At one time, the Marxist far left, and to a lesser degree, the anarchists, had a significant presence in working class communities and working class organizations, but they're no longer a factor. Most people who support the organizations we support would nowadays describe themselves as reformers rather than revolutionaries, as "liberals" or "progressives", rather than as socialists, and would be concentrated overwhelmingly in the Democratic party in the US and the social democratic parties in other countries. Those who are disgruntled with their leaders, I'd describe as "left liberal", for want of a better term - the milieu around the Nation, Huffington Post, MoveOn, Daily Kos, etc.
They definitely seem more numerous, better organized, and more assertive than during the Clinton period - Bush and the internet have probably been the two most important contributing factors - and, while I can't speak for Julio, I expect that's who he has in mind.