If you are one of those people — and there are a lot of you — who think today’s election is a referendum on President Bush’s conduct of foreign policy, in Iraq in particular, you are right. But if you are also one of those people — and there are some of you — who think that Tuesday’s results will have an impact on that foreign policy, then you are probably wrong.

If the Democrats win either the House or Senate (or both), there should be little doubt as to the reason for their victory. In a time of war, the conduct of American foreign policy becomes the electorate’s chief concern. Recently pollsters for ABC News / Washington Post asked respondents which issue will be foremost in their minds when they cast their vote. Iraq topped the list at 31 percent. Add to this the percentage of respondents who said terrorism would be foremost (11 percent), and a plurality of those surveyed named foreign affairs as the most important issue in this election. Last week pollsters for Newsweek asked respondents the same question. In the Newsweek poll, Iraq also topped the list at 32 percent, and terrorism came in third at 12 percent. That’s 44 percent of respondents who named matters dealing with foreign policy as the most important issue of the 2006 election.

This is the third election since the war on terrorism began, for the United States, on September 11, 2001. In the previous two elections, the prominence of national security issues has helped the Republicans. But times have changed. National security and American foreign policy still dominate our politics, but the Republican advantage on those issues has almost been erased. And you most likely already know why. The lack of a clear victory in Iraq has soured the public on President Bush and his perceived enablers in the Republican Congress. Last week’s New York Times poll was especially revealing. Not only did respondents cite Iraq as the single most important issue in this election, they also seem to have repudiated Bush policy. Only 20 percent of respondents in the Times poll said the United States was winning in Iraq. Just under a third of respondents approved of the manner in which Bush has conducted Iraq policy.

All year long, Democrats have portrayed the midterm elections as a referendum on Bush while Republicans have emphasized the costs (in their view) of Democratic majorities in Congress. Because the electorate so strongly disapproves of the conduct of the war in Iraq, it looks as if the Democratic portrait of the election will most closely reflect Tuesday night’s returns. The problem for Democrats is that while they will almost certainly benefit politically from Bush’s failures in Iraq, they will also have almost no power to change things.

The structure of American constitutional government entrusts the presidency with primary authority over foreign affairs. The upper house of the legislature has the power to ratify treaties and both houses exercise power over the purse. In the past, Democrats have used that power to cut off funding for unpopular wars such as Vietnam. But the Democrats I talk to say it is unlikely Nancy Pelosi would choose to fight such a divisive battle with President Bush. It is more likely House Democrats would put to a vote a resolution proposing a timetable (without any specific times) for American withdrawal from Iraq. Let’s say it passed. How would Bush respond? He’d ignore it.

There are two ways Congressional Democrats might be able to influence American foreign policy. The first is that, if Democrats gain a majority in the House, they will begin to launch investigations into war contracts and waste in military spending. And they will also be sure to harangue administration officials whenever one shows up for a hearing. Democrats can be nosy, and they can be loud.

Second, if Democrats win big tonight, it will become more and more difficult for the administration to avoid the fact that its current policies in Iraq have brought it nothing but controversy and unpopularity. If Bush has any chance to rescue his reputation, it lies in adapting to realities on the ground — perhaps by sending more troops to buttress the Maliki government — and changing the cast of characters in Washington, perhaps by letting Defense Secretary Donald Rumsfeld resign.

In the end, though, voters hoping for change must recognize that in matters of foreign policy, it is the president, for better and for worse, who is the ultimate decider.