There is great promise in the presidency of George W. Bush as he finishes his first 100 days in office April 30. But let there be no mistake about it: His handling of the abortion question, in his Supreme Court nominations and in his legislative agenda, will determine whether history considers Bush a great president or a mediocre one.

America's struggle with slavery is a good comparison with its current battle over abortion. Slavery was forbidden by our nation's founding principles (“all Men are created equal …”), it had a polarizing effect on the nation and was tolerated by our nation's leaders for political reasons. Abortion is also forbidden by our Declaration of Independence (“… and endowed by their Creator with … certain unalienable Rights, that among these are Life”), attracts strident foes and supporters and is tolerated for fear of angering voters.

Slavery shapes the way we evaluate past presidents. Small children to this day can often name only two presidents: George Washington, because he was first, and Abraham Lincoln, because he was great.

Bush could be added to that list, if he handles the abortion issue right.

That's because it's virtually certain that, in years to come, abortion will be remembered with every bit the horror (and more) that we associate with slavery today. Science tells us more every day about the lives of the unborn. With developments like 3-D ultrasound, it will be increasingly difficult to convince the public that abortion is anything but the killing of one's children. And psychologists face the terrible damage done to abortion's other victims: the mothers who consent to it, often under intense pressure.

Bush should model his strategy on the way Lincoln handled slavery.

Lincoln always took into account the paradox of a public which 1.) had an innate moral sense that knew slavery was wrong and 2.) had an unfair but real distaste for the abolitionists who raised that moral outrage in public.

Lincoln's public statements reassured audiences that he understood their dislike of the issue — and even made that dislike itself into another reason to oppose slavery.

In an 1854 speech, he cleverly expressed his outrage over slavery as wounded patriotic pride: “I hate it because of the monstrous injustice of slavery itself. I hate it because it deprives our republican example of its just influence in the world — enables the enemies of free institutions, with plausibility, to taunt us as hypocrites — causes the real friends of freedom to doubt our sincerity, and especially because it forces so many really good men amongst ourselves into an open war with the very fundamental principles of civil liberty — criticizing the Declaration of Independence, and insisting that there is no right principle of action but self-interest.”

Lincoln refused to make himself “better” than slavery's supporters. “They are just what we would be in their situation,” he said of Southerners.

Bush could do the same. He could argue that America ought not give China the opportunity to call us hypocrites on human rights. He could ask “pro-choice” voters to extend choices even to the unborn. He should always tell mothers under pressure that he understands their pain and wants to help.

Lincoln also spelled out what was ultimately at work in the slavery debate: “It is the eternal struggle between these two principles — right and wrong — throughout the world,” he said in 1858. “They are the two principles that have stood face to face from the beginning of time; and will ever continue to struggle.”

They stand face to face today.

Which side will George W. Bush be on?