We have become accustomed in the six years of the George W. Bush presidency to seeing issues split the parties and the nation down the middle, with almost all Republicans on one side and almost all Democrats on the other.

There have been exceptions (the 2002 education law), but this has been the pattern on tax cuts, health savings accounts, trade-promotion authority and free-trade agreements. Immigration is different. This is an issue that splits both parties, the Republicans most visibly, but the Democrats, too. It is an issue on which politicians, being what they are, seek political advantage, but it is also an issue where they're not quite sure where the political advantage lies.

No wonder Congress and the White House didn't bring the issue forward till the fifth year of Bush's presidency, even though he campaigned on it in 2000. And no wonder that members of Congress keep shifting positions on the issue. The House passed a border-security bill, with no guest-worker or legalization provisions, last December. Last month, the Senate Judiciary Committee approved a bill touching on all three issues. But Minority Leader Harry Reid kept the bill off the floor. The issue seemed dead on Capitol Hill.

Now it may be coming alive again. Speaker Dennis Hastert seemed obdurate earlier this year in his opposition to any bill with guest-worker and legalization provisions. More recently, House Republican leaders have said they might consider such provisions in conference committee. Reid, having declared the bill dead, now says he might let it come to the floor with some amendments -- which is what he did after claiming to have "killed" the Patriot Act last year. Majority Leader Bill Frist, who sponsored a border-security-only bill, says he'll allow a more comprehensive bill on the floor.

You can see political calculation in all this. Hastert and Frist can see the long-term danger for Republicans in seeming hardhearted against Hispanics, the fastest-growing segment of the electorate. Reid and Democrats can see the short-term danger in being viewed as killing a border-security law. Leaders in both parties don't want to be perceived as knuckling under to demonstrators brandishing Mexican flags. But they also don't want to be seen as continuing to ignore the fact that, on their watch, the border has become a sieve.

Republicans face the bigger political split on immigration. A large part of their base feels strongly about the issue and wants border security beefed up and immigration cut. But Democrats are split, too. Part of their base -- including many black politicians and voters -- sees immigrants as competitors for low-skill jobs. Most Democratic politicians have been willing to support generous guest-worker and legalization provisions. But not all their base is on board.

A columnist is tempted to say that the politicians should toss aside political concerns and do what they believe is in the public interest. Easy enough to say. But something just like that may be happening. Politicians act out of some combination of calculation and conviction -- the proportions vary. On immigration, there are some politicians, of both parties and on both sides, who are visibly acting out of conviction. And not just the noisy immigration restrictionists, like Rep. Tom Tancredo, who wants a border fence.

These conviction politicians include Sens. Edward Kennedy and John McCain, who favor relatively generous guest-worker and legalization provisions, and Sens. Jon Kyl and John Cornyn, who favor a less generous version. Add to this list George W. Bush, who seems poised to take an unusually active role on the issue.

The route to agreement is to give all of these conviction politicians much of what they want. A fence, high-tech border-security and identification devices, some compromise on guest workers and legalization -- all could be part of an omnibus measure. As for the calculation politicians, as they try to assess the political landscape and reconcile the seemingly contradictory findings of various polls, they appear to be coming to the conclusion that inaction -- or blocking action now that the issue is so visible -- poses a higher political risk than taking action.

Voters understandably believe we should have better border security and should do something about the 12 million illegal immigrants in our midst. Neither Congress nor President Bush has acted in five years. Maybe, just maybe, they're on the brink of doing so now.