Who were the ''Populists''? Rep. Tom Harkin (D) of Iowa, an eight-year veteran of Congress, and 13 other liberal members have just decided to revive the name in a legislative group they call the ''Populist Caucus.'' Shades of Ignatius Donnelly, the firebrand from Minnesota; of Jerry Simpson, the ''sockless Socrates''; of Mary Lease, the ''Kansas Pythoness''; of Gov. John P. Altgeld of Illinois who pardoned the anarchists; of William Jennings Bryan himself, who took all the aid from Populists he could get, grabbed the Democratic presidential nomination in 1896 with a single ''Cross of Gold'' speech, and became virtually undisputed leader of the Democratic Party until 1912. Those were gaudy days. The word ''Populist'' had magic then.
Always in the East there was the feeling among respectable people that this agrarian revolt was dangerous. In a way, of course, it was. With a smile Congressman Harkin gave me a replica of the 1892 depression-year platform which began with the trenchant statement: ''The railroad corporations will either own the people or the people must own the railroads.''
The farmers were troubled with debts and high interest rates; they asked for currency inflation - free and unlimited coinage of silver (a ratio of 16 to 1); they wanted aid to farmers against foreclosures; an eight-hour day; restriction of immigration; popular election of US senators; the initiative and recall, and - oh, horror - a graduated income tax! Congress actually enacted the radical income tax thing (2 percent on incomes above $4,000). The Supreme Court took one look at it and called it unconstitutional. That is why we have the 30-word 16th Amendment in the Constitution today - ratified in 1913 - to cancel that particular judicial ukase.
What do Mr. Harkin and his supporters seek now? He is an alert, concerned, earnest legislator. He believes, in brief, that there is less democracy in government than there ought to be. Look at today's farm foreclosures, at the giveaway prices of commodities, at corporate mergers, at special-interest lobbies.
The new ''Populist Caucus'' has got out a 1983 statement. It begins:
''One hundred years ago, a movement of ordinary citizens, of farmers, small business people and laborers formed in the West and the South. It was a movement of 'common people,' a movement to return control to the people of government and their daily lives. It was a movement that believed the corporate and moneyed elites which then dominated the government and the economic fabric of this nation had no right to determine the destiny of millions of Americans. It was a movement to restore to the government and to the society the principles of economic independence and equity. It was a movement to return economic power to the common citizen.''
And so on. Who can contradict these goals? America sees a revival of Populist protest with every period of economic hard times. The political problem is to get these diverse elements to cohere. Mr. Harkin, for example, tries to draw a distinction between ''liberals'' and ''Populists.'' He uses the illustration of food stamps; a ''liberal'' approach, he says, is to take care of those who can't afford to buy food stamps. But the ''Populists,'' by contrast, according to his conception want to attack the system itself: to ''build an economy in which everybody works and there is no need, and people get paid money instead of stamps.''
What Mr. Harkin's movement, in other words, wants to do is to create an ideology that its supporters can follow. That has been one of the hardest things to accomplish in the history of American politics. Socialists never got very far in the US, let alone the Communists. Partisans think they perceive a broad general difference between the ''Republicans'' and ''Democrats'' but even here nobody is ever quite sure what it is. Rep. Phil Gramm of Texas has just switched parties, and now is a Republican. Sen. Wayne Morse switched the other way, a couple of generations back.
It is easy enough for the new Harkin group to say that the government should ''harness the excessive power of wealthy, special-interest groups over the political and economic system.'' One man did that: It was Franklin Roosevelt who came to office here just 50 years ago, March 4, 1933. A quarter to a third of all the nation's working force was unemployed. It was easier for FDR to shake things up in such a disaster.
The new Populist Caucus promises to take a ''hard look'' at natural gas prices, high taxation, and foreclosure of farm mortgages. And it goes back to that angry old issue, the income tax. It asserts flatly that the progressive tax code is ''riddled with loopholes'' and that ''giant corporations pay little, if any, taxes.'' A century or so ago the Populists helped originate this reform; now a little band of dedicated descendants is riding out to see what has happened to it.