When I think of 'Left Wing' I'm thinking of the historical social concepts stemming from the french revolution: equality, fraternity and solidarity.

Those concepts have been most politically supported by the democratic party thru much of the 19th and 20th, no?. Not always or evenly thru out the country but generally speaking. The south gets complicated. A distrust of banks, elites and the professional class and support of working class and labor as a traditional base has been, until Clinton, main characteristics of the party.
I've agreed with Franks history of analysis of the Democratic party, as well as Chomsky, Hedges, Zinn and Vidal. The party got into a crises in the 70's and lurched to the right in the panic of 3 presidential election defeats.
Today we have a political spectrum that is center right democrats always groping for a public/private solution to governing and a deaf ear towards Labor. Deep in the pocket of the FIRE economy and pandering to a preferred college educated professional credentialed republican voters in the vain attempt at peeling some off for narrow margin victories. Always coming up short and alienating more and more of their traditional working class base. A strategy that came up short in 2016.
I'm not sure who the Democratic party represents anymore as they are decidedly NOT interested in egalitarian causes. Adopting instead a 'merit' based system.
Their lack of solidarity with Labor was also on display in 2016 and the decades before. Nafta, card check, ignoring the rank and file while campaigning, etc...
Liberty? Think massive expansion of the prison system and the Clinton crime bill.
Equality? Inability to raise the minimum wage. Clintons repeal of Glass Steagal. Saving the banks while millions lost homes.

To be sure the republican party is now a party of racists and xenophobes. That's all they really have on tap to offer. Owning Libs by throwing revanchism in their face. I agree with Frank that the republicans have stolen the language of populism out from under democrats (And used effectively by Trump where it mattered). Nut why not? Democratic leaders have long forgotten it's meaning and why.

I don't think you can claim regulation as a litmus test for wether or not a political party is left or right, JGW. I'd argue that some decent regulation came out of Busch Sr.'s administration. Most notably after the S&L crises. (remember how they actually jailed white collar criminals then?). They didn't 'foam the runway' for them as Obama did. See the contradiction? The right can regulate and has done so in the past. It's not a valid argument to say that regulation belongs solely to a 'left' political party. Regulations often are done at the behest of monied interest to create barriers of entry to markets and reduce competition, etc..

I still say there is no left political wing in American politics currently though I hope that changes for the good of the country.