As the Democratic presidential race finally gets down to brass tacks, two issues are becoming paramount. But only one of them is clearly on the table.

That is the issue of illegal immigration. A very smart Democrat, a veteran of the Clinton administration, told me he expects it to be a key part of any Republican campaign and is worried about his own party’s ability to respond.

I think he has good reason. The failure of the Democratic Congress, like its Republican predecessor, to enact comprehensive immigration reform, including improved border security, has left individual states and local communities struggling with the problem. Some are showing a high degree of tolerance and flexibility. Others are being more punitive. But all of them are running into controversy.

I noticed a new Siena College Research Institute poll of registered voters in New York. It found heavy opposition to Gov. Eliot Spitzer’s proposal to permit undocumented aliens to obtain driver’s licenses; nearly two-thirds opposed the latest version.

Moreover, the issue is part of a weakening of support for Spitzer, who now has a 2-to-1 negative job rating and, for the first time, an overall unfavorable image. Asked if they are inclined to support him for re-election in 2010, only 25 percent say yes, while 49 percent say they would prefer an anonymous “someone else.” It was just last year that Spitzer was elected in a landslide. On Wednesday, Spitzer announced that he was abandoning the driver’s license idea.

That is New York, home state of both Hillary Clinton and Rudolph Giuliani. And the driver’s license question is the one that tripped up Sen. Clinton when she was asked about it at the Philadelphia debate and gave answers that were indecisive – and nearly indecipherable.

The other candidates had more time to compose an answer, so were spared the embarrassment. It was the pummeling she received from Barack Obama and John Edwards during and after that debate (and from moderator Tim Russert) that brought her husband, former President Bill Clinton, into the campaign, with the charge, as he put it, that “those boys have been getting tough on her lately.”

The former president’s intervention – volunteered during a campaign appearance on her behalf in South Carolina – raised the second, and largely unspoken, issue identified by my friend from the Clinton administration: the two-headed campaign and the prospect of a dual presidency.

In his view, which I share, this is a prospect that will test the tolerance of the American people far more severely than the possibility of the first woman president – or, for that mater, the first black president.

As my friend says, “there is nothing in American constitutional or political theory to account for the role of a former president, still energetic and active and full of ideas, occupying the White House with the current president.”

No precedent exists for such an arrangement and no ground rules have been – or likely can be – written. When Bill Clinton was president, the large policy enterprise that was entrusted to the first lady – health care reform – crashed in ruins.

The causes were complex, and some of the burden falls on other people – Republicans and Democrats in Congress, the interest groups and, yes, the press. But as one who reported and wrote in excessive detail and length about that whole enterprise, I can also tell you that the awkwardness of having an unelected but uniquely influential partner of the president in charge affected every step of the process, from the gestation of the plan to its final demise. She was never again asked to take on such a project.

And this was simply the confusion sown by having the first lady in charge. Put the former president into the picture – however “sanitized” or insulated his role is supposed to be – and the dimensions of the problem loom even larger.

No one who has read or studied the large literature of memoirs and biographies of the Clintons and their circle can doubt the intimacy and the mutual dependency of their political and personal partnership.

No one can reasonably expect that partnership to end, should she be elected president. But the country must decide whether it is comfortable with such a sharing of the power and authority of the highest office in the land.

It is a difficult question for any of the Democratic rivals to raise.

But it lingers, even if unasked.