On Monday the U.S. Senate will take up S. 2230, “The Paying a Fair Share Act of 2012,” which would enact the so-called “Buffett Rule.”

Senate Majority Whip Dick Durbin, D-Ill., admitted to us on Wednesday that S. 2230 has no chance of getting the 60 votes it needs to close off debate and pass in the Senate. Nevertheless, Durbin told a meeting of St. Louis Post-Dispatch editors and reporters, “If you believe in progressive taxation, this is fundamentally fair.”

The Republican presidential primaries demonstrated that many GOP voters, even those who aren’t millionaires, aren’t sold on the concept of progressive taxation, which holds that as one’s share of society’s benefits increases, so should one’s share of society’s costs.

While S. 2230 might fail on Monday, the “Buffett Rule” will be back again, attached to other legislation that will be considered before the November election. Democrats see the issues of tax fairness and income inequality as political winners — particularly as Mitt Romney, the putative GOP presidential nominee, paid a tax rate of only 13.9 percent on $21.7 million in income in 2010.

That was the issue that billionaire investor Warren Buffett addressed in a controversial op-ed article in the New York Times last August. He argued that it was unfair that wealthy investors, including him, can pay lower rates than middle- and upper-middle-class taxpayers pay in combined payroll and income taxes.

Buffett said, for “those making more than $1 million . . . I would raise rates immediately on taxable income in excess of $1 million, including, of course, dividends and capital gains.”

In our meeting with Durbin, he told us that enacting the Buffett Rule in S. 2230 would raise $48 billion over 10 years.

Sen. Sheldon Whitehouse, D-R.I., sponsor of S. 2230, told the Washington Post’s Ezra Klein that his bill could raise considerably more than that. The “millionaire’s tax” would phase in at incomes higher than $1 million and not fully apply at 30 percent until the 2-millionth dollar in income.

If you assume that all the Bush tax cuts expire as scheduled at the end of this year, the “Buffett Rule” would raise $47 billion. But if you assume, as the Republican House budget does, that all the tax cuts are extended, Whitehouse’s bill would raise $160 billion.

Right now these are political talking points, and they are likely to remain so until after November. The important issue is the principle:

What is a fair share, and who should pay it?