The difference between liberalism and progressivism is ideological.
Historically, liberalism started with John Locke, whose philosophy is here superbly summarized, explained, and referenced to its sources, as I shall place in quotes from that article:
Locke, writing in 1689, “praises money as probably no one prior and after him,” and “The importance of property is so high according to Locke that he makes its preservation ‘the great and chief end’ [“The great and chief end, therefore, of men’s uniting into commonwealths, and putting themselves under government, is the preservation of their property.”, Ch. IX, Sec. 124] for which men unite into commonwealth and establish government.” Lord Acton, writing in 1877 criticized Locke’s philosophy because Locke’s “notion of liberty involves nothing more spiritual than the security of property, and is consistent with slavery and persecution.” That was the case because Locke’s philosophy wouldn’t have gotten any aristocratic sponsors at all and would therefore simply have failed utterly and become lost to history, if it didn’t allow slavery, and allow unequal rights that depend upon the possession of wealth and respecting the wealthy more than the poor (and with slaves being at the bottom). However, Lord Acton was himself passionately committed to America’s southern states’ Confederacy during the Civil War, because he believed more in the principle of local or home rule (as opposed to federal rule) than he opposed “slavery and persecution” (such as against slaves). So: the entire aristocracy except for the outright conservatives amongst them were hypocrites, and liberalism is based on that hypocrisy, nothing which is internally consistent. It’s pure hypocrisy and therefore sloppy thinking, because otherwise it would be simply “Might makes right” — pure conservatism — unadorned with any pretenses to decency, and to equality of rights irrespective of wealth.
By contrast, progressivism is a total and unequivocal rejection of conservatism. It doesn’t respect wealth and the wealthy, nor does it despise the poor; it instead admires goodness which seeks no recompense or public honor therefore, but which springs instead from as wide a compassion as possible (without regard to an individual’s race, creed, or ethnicity), and which despises greed, and rejects outright the “Greed is good” philosophy, and accords no honor to winning and no shame to losing. The progressive (like anyone) wants to win, but only for a good cause, as “good” is accorded without any favoritism toward any individual or group or individuals, nor disfavoring of others, but purely with the equal rights of all, to freedom and happiness and fulfillment.
However, because progressives are committed to the good, they do passionately support the good against the evil, and, to them, the “good” are progressives, and the “evil” are proponents of might-makes-right. This is what it’s all about — the difference between progressivism and conservatism. Liberalism is merely in-between.
Might-makes-right — the very core of conservatism — is automatically favoring the rich against the poor, because money is power and the poor are therefore the weakest class of all. Locke’s First Treatise on Government, “Book I. Ch. XI. Who Heir?” (Sec. 107) goes even further into its conservative tradition of might-makes-right. It asserts that “the author” of the Bible is “affirming that the assignment of civil power is by divine institution, … so that no consideration, no act or art of man, can divert it from that person, to whom, by this divine right, it is assigned.” Liberalism not only accepts hereditary rule, but it rejects any sort of democracy, because for a liberal, accountability is only to The Almighty, not to any mortal nor public of mortals. Thus, “it would be as much sacrilege for any one to be king, who was not Adam’s heir, as it would have been amongst the Jews,” because the will of The Almighty is eternal, and not merely in the past. When America’s Founders refused even so much as to mention any god, and opened their Constitution with “We, the people of the United States, … do ordain and establish this Constitution,” they were repudiating not only conservatism but liberalism, and starting progressivism, because accountability here was thus being ordained, in this new country, to be accountability to the mortals, the public, and not to any The Almighty, at all.
Because a progressive favors equally the rights of all, and rejects unconditionally the supremacy of any nation above any other, a fundamental principle of progressivism is an utter rejection of imperialism, and of any invasion of one country by another which has not become, nor been reasonably considered to pose a danger to become, invaded by that other, which it has invaded — it has committed aggression against that country. Consequently, for example: America, which since 1898 has unprovokedly invaded and sought to vassalize and exploit, more countries than any other, is perhaps the most conservative country, at least since around 1900. The US is, in this sense, profoundly anti-progressive, because it has become the most imperialistic nation in the world, having taken the place of England, and of France, and of Spain, and of Germany, and of Portugal, and of Japan, and of Italy.
Consequently, to be a progressive in today’s America is to be ideologically rather isolated, alone with the Founders, in this now extremist conservative society, so alien to its own Constitution. It’s to be at a severe disadvantage, in this society. But, to a progressive, there is no assumption that the good will be rewarded and that the evil will be punished. There is a hope for that, but no expectation of it. Consequently, the progressive is a progressive not for personal gain but purely to do justice in the world. Recognizing that the world’s norm is profoundly unjust, a progressive seeks especially to aid the good against the evil, irrespective of the consequences to oneself.
Obviously, therefore, there are far more liberals than there are progressives. It’s not profitable to be progressive, but it is good to be progressive.
Locke, having been a liberal instead of a pure conservative, would not approve of today’s America, because by his basing his ethic upon property (wealth), he acknowledged the extreme evilness of conquest. Thus, “Book II. Ch. XVI. Of Conquest” in his classic, Second Treatise on Government, states (Sec. 176):
That the aggressor, who puts himself into the state of war with another, and unjustly invades another man’s right, can, by such an unjust war, never come to have a right over the conquered, will be easily agreed by all men, who will not think, that robbers and pirates have a right of empire over whomsoever they have force enough to master; or that men are bound by promises, which unlawful force extorts from them. Should a robber break into my house, and with a dagger at my throat make me seal deeds to convey my estate to him, would this give him any title? Just such a title, by his sword, has an unjust conqueror, who forces me into submission. The injury and the crime is equal, whether committed by the wearer of a crown, or some petty villain. The title of the offender, and the number of his followers, make no difference in the offence, unless it be to aggravate it. The only difference is, great robbers punish little ones, to keep them in their obedience; but the great ones are rewarded with laurels and triumphs, because they are too big for the weak hands of justice in this world, and have the power in their own possession, which should punish offenders.
So: today’s USA stands condemned even in that compromised work. This is because John Locke wasn’t a pure conservative — in that passage, he clearly condemned might-makes-right, which he said pertained “in this world,” meaning not in the world of The Almighty, in a supposed afterlife, where power, supposedly, does reflect virtue. By contrast, a progressive makes no assumption that power reflects virtue anywhere. The only reason for a progressive to be virtuous is to be virtuous — and not according to any ‘The Almighty’, but instead recognizing, from the progressive’s experience of this world, that if this world does reflect, in any sense, the will of ‘The Almighty’, then that Almighty is anything but virtuous — certainly not, at all. It’s not a progressive world — not at all. To be a progressive is to accept this fact, not to be deluded that there is some reward in ‘an afterlife’, because a progressive doesn’t expect to be rewarded for virtue, but, more likely, to be punished for it. To do something for a reward is commerce; it’s no political ideology, at all. So, again: there are very few progressives.