The first group is made up of people who, for whatever reason, will not be able to make their monthly payments. Some took out mortgages with initial monthly payments that they couldn’t afford. Others took out adjustable-rate mortgages whose monthly payments have ballooned to an unaffordable level. Still others have lost their jobs.

At the start of this month, almost 1.5 million homeowners  out of about 75 million nationwide  were in this category. They were at least two months behind on their mortgage payments. Mark Zandi, the chief economist of Moody’s Economy.com, estimates that another five million or so will fall into the category over the life of their mortgage, as the economy worsens and more adjustable-rate loans reset.

The second group is quite different. It is made up of people who are at risk of foreclosure not because they won’t be able to keep up with their monthly payments  but because they may decide they don’t want to continue making them. These are the homeowners who are “under water,” which is to say their houses have lost so much value that they’re now worth less than the underlying mortgage.

Homeowners with an underwater mortgage face a choice. Many will stay put and keep making their monthly payments, because they see their house primarily as a home, rather than an investment. Maybe they love their neighborhood or their children’s school. Maybe they just don’t like the idea of reneging on a deal, as Brett Barry, a real estate agent near Phoenix, put it.

Others, though, are going to look at their home purely in economic terms and see an investment that may never pay off. Some of them will choose to walk away.

What matters, for the purposes of a bailout, is that the number of underwater homeowners is much larger than the number of people who will be unable to make their mortgage payments. Assuming that home prices still have a ways to fall, something like 19 million homeowners may be under water by 2010 (only a few million of whom will be struggling to make their payments).

Now, who should be rescued?

Let’s start by acknowledging that morality cannot be the main criteria, unfortunately.