1 > 50
While liberals would have you believe their universalism is the child of wholly benevolent aspirations, there is a good practical reason they oppose States Rights: they hate competition.
One example is Right to Work laws. Liberal supporters of unions claim to be against RTW because they’re looking out for the little guy, the working man, for whom RTW laws would be a soul-crushing blow and a lifetime sentence of poverty. When state-level democracy trumps union democracy, pro-union liberals invariably assert that it’s the end of the middle class as we know it. Politics of Fear 101.
Note that supporters of States Rights are just fine with different laws for different states. If one state wants forced unionism, let them have it. (I’m imagining right now some liberal coming up with a snappy NIMBY retort chock full of union rat imagery.)
1
The reason pro-union liberals don’t like States Rights is at least twofold. First, while multiple jurisdictions are in place, people can compare legal systems and test liberal claims that everything they do is best. Liberals hate that.
It was Marx himself who bemoaned the plenitude provided by free trade as opposed to socialism. If only free trade were outlawed, socialists could claim theirs is the best of all possible worlds and what would anyone have to compare it to?
But when people can discern differences among systems, they choose which they want for themselves. Under a completely federalized system, liberals would attempt to monopolize the legal code. Once successful, they’d claim their laws were a Panglossian monopoly (e.g. public education), and to even question getting rid of the monopoly is an affront to human rights.
To sum up, they hate competition because it denies them the ability to portray their type of government as a superlatively beneficent monopoly for which there can be no alternative lest we all perish aimless idiots in a rising conflagration of our own stupidity.
Every good economist knows to consider both seen and unseen. When liberals manage to force choice  into being unseen by instituting a government monopoly, it becomes much harder to dismiss what remains to be seen.
2
Second, RTW laws mean a decrease in political cronyism between politicians and unions. (Oh, right, that doesn’t really exist. Everyone’s just making it up because they hate the working class.)
We saw this with the Boeing unions. The federal government is trying to prevent Boeing from opening a factory in a RTW state because it alleges doing so would amount to “retaliation” against union workers. For what? Who knows?
Liberals need to crush competition in legal jurisdictions to make sure their political allies can have as much power as possible. If a community of people doesn’t want forced unionism, to hell with that, we’ll force them to have forced unionism.
Race to the Race Card
Inevitably, an astute, longtime and dedicated viewer of MSNBC will posit the only logical outcome of a plurality of jurisdictions will be a return to slavery itself. For weren’t States Rights the rallying cry of the dirty, Southern slavers during the heretical rebellion they perpetrated?
If states could write their own civil rights laws, some would be free, sure, but others would be Jim Crow revivals. But this is just another typical left-wing smear dressed up as political theory. The premise, of course, is that States Rights advocates are all closeted racists, and liberals are the only thing preventing the establishment of a Fourth Reich in the USA.
By the way, did anyone else hear about Rand Paul or any other tea party member of congress working to repeal the Civil Rights Act of 1964? Me neither. Nevertheless, as MSNBC portrayed it in October 2010, the whole country was about to re-enslave anyone darker than Mitt Romney. Never mind that tea partiers were talking fiscal policy. Liberals needed their straw men racists so they could paint themselves as the saviors of hyphenated-Americans everywhere.
The Big 1
I’m quite certain most liberals would prefer the UN be in charge of taxation. A variety of tax jurisdictions and regulatory climes makes preventing capital flight very difficult for an aspiring American socialist. They need to pen in their tax cows so there is no escape. So much for a voluntary social contract. What’s a liberal to do?
I know. Claim lowering tax rates is the product of selfishness perpetrated by the rich and enabled by republicans. That’s right, it’s selfish to keep your own money, but not so when you want to take someone else’s while giving nothing of your own in return.
This is why all conservatives should support States Rights and the concept of Nullification: they are the keys to preventing liberals from monopolizing government and crushing any remnants of choice we have left. As all conservatives should know, liberals demonize and politically marginalize anyone who even dares consider a right to choose something other than a government monopoly exists after all. So pick up the slack, conservatives. The choice is yours.