Mrs. Clinton has also made clear what she wouldn’t do. She would not support reforms that require low- and middle-income people to pay more or accept less. She would not divert Social Security payroll taxes into private investment accounts, a favorite idea among Republicans. She would not lower Social Security’s cost-of-living adjustment, or raise the retirement age, currently 67 for people born in 1960 or later. (Both ideas have broad Republican support and narrow Democratic support.)

She is right on all counts. Privatization would weaken Social Security while exposing retirees to the risks of losing money in the stock market and outliving their savings. There is no compelling evidence that the current cost-of-living adjustment is too high. Raising the retirement age for everyone — an idea based on the simplistic assertion that everyone is living longer — would mostly harm lower-income workers because the wealthy are increasingly outliving the poor.

Despite those lines in the sand, Mrs. Clinton’s plan allows for compromise with Republicans who favor cutting benefits to keep the system viable. Her opposition to raising the retirement age across the board, for example, does not preclude benefit cuts for high-income recipients. This fix can be made simply by adjusting the formula used to calculate benefits for high earners.

Donald Trump has said he would not cut benefits. But it’s fair to ask whether he can honor this pledge while at the same time honoring two other promises — to slash taxes and raise military spending — that, together, would lead to explosive federal deficits unless there were spending cuts in large programs like Social Security. And Mr. Trump has dodged the solvency issue. Absent benefit cuts, the only sure way to shore up the system’s finances is to increase revenue, an option that Mr. Trump has never broached.

Mr. Trump’s approach — no benefits cuts and no tax increases — closes off both avenues to solvency. Which, in the long run, would essentially bankrupt the system.