Twenty years ago this week, President Bill Clinton signed welfare reform into law — the most revolutionary domestic political achievement in decades.

With the law, Uncle Sam took a giant step toward ending the decades-old cycle of long-term dependency that old-school welfare had caused. In its place came a new emphasis on moving welfare recipients into the workforce, giving them a real sense of self-respect and financial independence.

It’s been one of America’s great success stories — yet it remains under attack from leftists who insist it did more harm than good and want to restore the old down-spiraling cycle of open-ended cash handouts.

A new analysis by the Manhattan Institute’s Scott Winship shows just how successful welfare reform has been — and how wrong the critics were then and are now.

Far from creating more poverty, Winship shows, the new rules have left children — particularly in single-parent families — less likely to be poor today. Indeed, poverty rates for black children and single-parent families have dropped to historic lows.

And as people moved from welfare to work, employment rates for single mothers rose 15 percent, while teen pregnancy took a dramatic drop.

Indeed, with the growth of non-cash government aid — food stamps, housing aid, Medicaid, etc. — “there is little evidence that welfare reform caused an increase in hardship or extreme-cash poverty,” the report concludes.

Yes, some problems remain: The growth of those non-cash programs, for example, tends to undermine work requirements.

But the underlying principle has been proved: Most people on welfare don’t want to remain forever dependent on government handouts. And the best way to fight poverty is to help them join the workforce.