From my perspective the opposite has happened. The entire political scale seems to have shifted to the right.
On which basis do you think the political scale in the US has shifted to the left? And as a corollary, do you think it’s done so in both economic and social ways?
For example, I consider myself a social liberal, fiscal conservative. Meaning I favour small government, fiscal restraint, and more traditional means of fiscal policy. However, I am not fussed by non-traditional gender norms, relationship models, access to abortions, and other socially progressive ideas. So in this shift left in your view, is it fiscally, socially, or both? And why?
In the US, the political scale has most certainly shifted to the left. Both fiscally and socially.
I think there can be little credible debate that it has not shifted socially. Regardless of whether you are "not fussed" or whatever your personal opinion, that social change has occurred. Marriage between homosexual partners, which would have been unthinkable 20 years ago, is essentially legalized across the country. There is an increasing erosion of Freedom of Speech, once a sacrosanct cornerstone of our political process, by labeling unwanted speech as "hate speech" and banning it or otherwise suppressing it. Look at the intolerance by the left towards any sort of conservative speakers (disrupting lectures, riots, etc.) on campus---those actions are also seen in the general society as these students move off campus and bring their paradigm to other areas of the body politic. Look how the change social symbolism has grown--from an understandable rejection of Confederate symbols, the needle has been moved so that big companies like Nike will extirpate the Betsy Ross/Revolutionary War flag for being "racist". Most mainstream media is vehemently anti-Trump and don't even make an effort to put up a pretense of being balanced any more, and that has increasingly extended to ANY political position they disagree with. It is no longer an opposite political opinion to them, it is a MORAL CRIME. By my way of seeing, this allows them to trample the rights of those they disagree with.
On the fiscal side, there has also been a steady march to the left. Sure, Trump was able to reduce the marginal tax rate, but for society as whole whole, there is an increasing percentage of the population who are embracing socialism or socialist policies. That has shown up in repeated surveys. (https://www.cnbc.com/2018/08/14/fewer-than-half-of-young-americans-are-positive-about-capitalism.html)
You might think that there is nothing wrong with being more socialist, but that's not the question being addressed. The question is "do you think the political scale is being shifted to the left" and the answer is yes, regardless of whether one thinks this is a good thing or bad thing. My own personal suspicion is that increasing fiscal inequity is contributing greatly to this movement, and that is largely driven by the severe disruption in the economy due to globalization and trade policies. (Paul Krugman, the left-wing economist and anti-Trumper wrote such an admission a couple of months ago: https://www.bloomberg.com/opinion/ar...990s-economics
I think I'll stop here, as this can easily develop into a thread of its own. But this is a long answer to your question of "whether the political scale has shifted to the left." Notice, it is NOT about a temporary shift in political power, as that is a very fickle thing. I'm talking about the political temperment of the US in both social and fiscal terms. Frankly, it is so obvious, I have to question either the knowledge of US society or the objectivity of someone who would deny it.