COLUMBUS, OHIO ? Ken Mehlman, the head of President Bush’s re-election campaign, is clearly right about one thing. “This is a big-issue election,” he said in an interview the other day. “We are not talking about trivial differences.”

The truth of that statement is amply proved by the speech that Bush gave on Monday night, kicking off his campaign, and the response heard Thursday night in the Los Angeles debate from the leading Democratic candidates, John Kerry and John Edwards.

The public recognition that the stakes are larger than usual explains why interest in Tuesday’s primaries here and in nine other states is greater than anyone expected. Even though the candidates have barely had time to touch down in New York, California, Ohio, Georgia, Maryland, Massachusetts, Minnesota and the three smaller states also balloting, expectations are for big turnouts.

Americans understand how much rides on the choice we will make. As the president said, “Great events will turn on this election. The man who sits in the Oval Office will set the course of the war on terror and the direction of our economy. The security and prosperity of America are at stake.”

That is more than rhetoric. Two factors have merged to make this election consequential — much more so than most second-term decisions usually entail. One is the emergence of genuinely new forces in the world and at home, demanding tough policy decisions. The other is the way Bush has responded — or failed to respond — to these changes.

The threat of terrorism, rooted in radical Islamic movements, challenges the entire structure of international relations and the stability of the worldwide balance of power. At the same time, the rapid spread of advanced technology into previously backward countries — notably China and India — has accelerated the pace of globalization and upset the economic stability of the industrial world.

Confronted with these unprecedented challenges, Bush has chosen a path of boldness and risk in one instance and of great caution — almost inaction — in the other. He has been as innovative in dealing with the security threat as he has been passive on the economic front. And in both areas, the Democrats are prepared to question his policy and leadership.

On the military-diplomatic front, Bush has declared a policy of pre-emptive action against any movement or government that he judges a threat to America. He has asserted — and in Iraq has demonstrated — a willingness to go it alone, arguing in his speech that “America must never outsource America’s national security decisions to the leaders of other governments.”

Although they initially voted to back his policy in Iraq, Kerry and Edwards have denounced its execution and have made it clear they do not accept it as a model for future actions. Instead, they want to return the United States to its traditional international status, as a leading figure in NATO and the United Nations, bound to and substantially influenced by other countries.

On the challenges presented by the new international economy, however, Bush’s basic response has been to continue and accelerate the liberal trade policies of the previous two decades. He has continued to reduce the main federal revenue resource, the income tax, while trusting that resulting growth will produce the needed jobs and finance what he hopes will be the declining domestic responsibilities of government.

After some initial hesitation, the leading Democrats now are in full cry against this policy. They would roll back some of the tax cuts, stiffen trade policy against foreign competition and greatly expand the federal role in providing health care, education benefits and retirement security.

Bush describes this domestic policy debate as one between “a government that encourages ownership and opportunity and responsibility or a government that takes your money and makes your choices.” Edwards — echoed by Kerry — describes it as a choice of confirming or rejecting a policy that produces deficits and unemployment, and divides the nation into “two Americas,” one for the wealthy and another for everyone else.

To this, one can also add the sharp divergence between the parties on social policy, whether it concerns abortion, homosexuals, guns or the proper relationship between church and state.

All of this — and more — is in play in the coming election. If we are lucky, the candidates will debate these issues with the seriousness they deserve. The country is ready for such an election.

David Broder is a columnist for Washington Post Writers Group.