True, but the post WW2 Democratic Party did a better job of it...
...until Civil Rights happened (among other things) and the New Deal coalition came apart. But looking into the future, however, Millennials and Gen Z are about 45% non-white. And since whites will probably continue to disproportionately populate the professional class, the power of racist whites to dominate working class politics will be diminished.
This is now a myth. Take a look at the detailed breakdown in some recent polling. Black voters are about 20% of the Democratic electorate, and about 20% of Sanders' supporters are black. Hispanic voters are MORE likely than the overall Democratic electorate to support Sanders. This is of course driven by the youngest age cohorts, which are both the most diverse, and also the most likely to support Sanders.
Progressives would also blame the moneyed interests for this. Pro-business candidates have an easier time fundraising (from wealthy and corporate donors), and the Party likes candidates that don't need as much help.
Tug it...toward where the party used to be historically? Tug it...toward actually representing the voters' opinions? But wait! Aren't more than half of Democrats likely to identify as 'moderate' or 'conservative'? Yeah, but self-id polls are pretty meaningless when it comes to determining actual policy. If you look at polls on individual issues, 2/3ds (or more) of Democratic voters support (in aggregate) a wealth tax, paid parental leave, univeral pre-K, M4A, Free College, a $15 minimum wage, overturning Citizens United, and worker co-determination. Some of these are even popular with Republicans. Granted, some have become fully mainstream within the party, but plenty haven't. So the goal of Progressives isn't just "pull the party to the left," it's "have a party that more accurately represents its voters." If the Democratic Party was actually doing a better job at this, Bernie (an independent) wouldn't have been able to go from a nobody to competing with the person that most Democrats had probably assumed they would be voting for since 2009.
Not to mention that Demographics and circumstances are highly likely to move the country to the left anyway, at least on most issues. Sanders and Warren's side clearly has ideological momentum and energy. Social conservatives are growing increasingly skeptical of the GoP's market dogmatism. Millennials are on pace to be the first modern generation to end up poorer than their parents. Millennials and Gen Z get to enjoy less wealth, higher healthcare, college, and housing costs, the 'joys' of the gig economy, not to mention the decades that have brought stagnant wages and near-record inequality, and all of the financial burdens previous generations are laying on them. The future is going to move left: but for now, the conflict at least as big as (if not bigger than) the Left/Center-left conflict is the generational one. Sanders and Warren are the most popular with the younger age cohorts (far more popular than the actual Millennial in the running), and the issues that drive their politics aren't going away any time soon.
I agree with the overall point, but Joe Manchin has said on record that he wouldn't vote for Bernie over Trump. He fails the most basic test- not just for a Democrat but for an American. So fuck Joe Manchin.
But my biggest problem with the Joe Manchins of the party isn't whether or not they have to skew conservative on particular issues, it's that they are the biggest recipients of Wall Street money. Voters are center-left on economic issues. The biggest unrepresented bloc of voters is those that are center-left to left economically but socially conservative. That should be the target for Red State Democrats, but that doesn't usually seem to be the case. The big donors certainly don't want that.
Give me fewer Joe Donnellys and more Sherrod Browns (who I think would've made a good candidate).
I wouldn't be so sure about his legacy.
For the reasons I described above, the Progressive left isn't going away any time soon. The actual organizing is just getting started. As the age of the Boomers fades, the influence of Millennials will increase. And again: their favorite candidate is Bernie Sanders. He's not exactly going to be forgotten. 20-30 years down the road, when some Millennial president finally signs a bill to deliver universal healthcare, they're going to name the damn bill (or at least something else) after him. So while lots of Progressives deeply want Sanders to be the next FDR, he won't be, due to the realities you have described- if he were to win, he would probably have a quite disappointing presidency. I do, however, think that he could be the next Barry Goldwater.
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For a while I've thought that if instead of letting the Congressional GoP set the agenda, he'd started with an infrastructure bill, his whole presidency would've been a lot better for him. He could've used the momentum of his surprise win and control over the GoP to hold their feet to the fire about actually spending some money, and he could've put Democrats in the uncomfortable position of having to choose between voting for popular legislation (that they want to do too) and a base that doesn't want to see them work with Trump.
But Trump being Trump, he was never going to do this.