Most presidential candidates are proposing some kind of health care overhaul, but when Congress failed to pass immigration reform this year it might have created a major roadblock to lawmakers’ ability to offer medical insurance to all.

So far, none of those proposing fixes to the health care system have said how they would treat illegal immigrants, understanding the furor the issue would cause on the campaign trail.

Yet, how any health care reform would treat illegal immigrants is almost certainly going to be a sticking point that would not have been the case had Congress already dealt with the overall immigration question.

Depending on who wins the Republican nomination, the issue could become a real dispute, because the general approach the major Democratic candidates have taken to such issues suggests they might favor including illegal immigrants in a health care overhaul.

Any such plan will face obstacles on Capitol Hill in 2009, no matter who wins the presidency. Even if the Democrats keep Congress and win the White House, agreement could be elusive. A divided government would make things even tougher.

There will be disagreements over the size of the government’s role in health care, the changes it will require from insurance companies and individuals and, most of all, the cost and scope of coverage.

But the question of how to deal with the estimated 12 million people here illegally could become a chasm too wide to bridge, especially once this becomes an issue in the race and positions become etched in stone, as is likely to be the case.

Simply put, the politics associated with providing taxpayer subsidies to illegal immigrants so they can buy health insurance might be a very tough sell to many members of Congress and their constituents, especially Republicans, who represent much of the Sun Belt, Midwest and Rocky Mountain states.

And a plan that does not offer coverage and subsidies to illegal immigrants might be unpalatable to many Democrats, especially those from the heavily blue states along the upper Atlantic and Pacific coasts.

The candidates trotting out their various reform plans are not offering the specificity required when their ideas are turned into legislation. But their proposals generally have some government subsidy or financial incentive for low- and moderate-income families to buy insurance.

About 10 million of the estimated 47 million U.S. residents without health insurance are thought to be illegal immigrants, the vast majority of whom would fall into the eligible income range. Currently, Medicaid, the federal health program for the poor, pays for emergency care for illegal immigrants, although in some states that has been used to cover treatment for non-emergency-room services such as chemotherapy.

It would be easy to imagine the political controversy over whether undocumented workers and their families would be eligible for the same incentives and subsidies as legal residents.

Immigrant advocates, some health care experts and many Democratic officials and interest groups would almost certainly argue that leaving illegal immigrants out of the proposal would be a public health disaster.

In that case, these experts predict the most severe cases among illegal immigrants would flood hospital emergency rooms that by law are required to treat everyone, driving up costs and leading to poorer overall health that could migrate to the general population.

But those who would oppose including illegal immigrants argue that scenario is actually the case now for all U.S. residents without health insurance, and reform without undocumented residents would cut that current problem by almost 75 percent.

These groups that stopped the immigration reform proposal advocated by President Bush and bipartisan congressional leaders might well be expected to balk at giving illegal aliens any taxpayer money.

It is worth noting that the question of whether to include illegal immigrants has been a major – although not the only – sticking point in California, where Gov. Arnold Schwarzenegger’s health care plan has stalled. And public opinion in California is probably more likely to favor such subsidies than it is in most states.

The outcome of the GOP presidential race will also be a factor. Former Gov. Mitt Romney and former Sen. Fred Thompson have both criticized front-runner Rudy Giuliani for lax enforcement of immigration laws while New York mayor.

If either Thompson or Romney wins the nomination, he might be more likely than Giuliani to make the status of illegal immigrants in any health care reform a campaign issue.

Like all things political, in health care reform, the devil is in the details. And this is one detail that could be very difficult to work out.