WASHINGTON, June 3— As the nation prepares to mark the 60th anniversary of D-Day, President Bush has steeped himself in the imagery and oratory of patriotism, military service and national strength.

But Senator John Kerry, his Democratic challenger, has vigorously contested that political ground.

Mindful of what happened in the 2002 midterm elections, when many Democrats felt Republicans utterly controlled the debate over national security, Mr. Kerry has used this two-week window around Memorial Day to introduce himself, aggressively, as an alternative commander in chief.

It is, in large part, a pre-emptive strategy. Mr. Kerry's advisers say they are well aware of the risks of challenging an incumbent president on foreign policy during a war. For days now, the Bush campaign has been accusing them, via surrogates and campaign advertising, of ''playing politics with national security.''

Geoffrey Garin, a Democratic pollster, asserted, ''The Bush administration has tried to use the theme of patriotism to counter any criticism of the war, including criticism from Senator Kerry.'' Mr. Garin added, ''They are trying to silence him.''

Instead, Mr. Kerry has outlined his vision of foreign policy, has criticized the administration's, and has defended his right to do so as in keeping with the nation's best patriotic values. He has delivered a series of major addresses on foreign policy, nuclear proliferation, bioterrorism and military modernization, which have often run with Mr. Bush's speeches in the news coverage.

Perhaps just as important, he has sought to demonstrate the more symbolic attributes of commander in chief. He has surrounded himself with veterans, proudly talked about his service in Vietnam, listened to the needs of the troops and their families and generally campaigned across the country as a man completely uncowed by -- in fact often dismissive of -- the current commander in chief.

''National security is a forefront issue in this campaign,'' said Tad Devine, a senior strategist for the Kerry campaign. ''And John Kerry is not prepared to cede an inch of it.'' Along the way, Democrats assert Mr. Kerry is also addressing the problem of every challenger to an incumbent president -- the stature gap. (Political, not physical.)

At nearly every Kerry rally in recent days, there have been flags and banners declaring some variation of ''Strength'' and ''Service.'' When he arrives in his new, red-white-and-blue campaign airplane, he descends to the tarmac with a sharp -- almost presidential -- salute to a welcoming party of local veterans, a popular shot for the local news.

Often, Mr. Kerry's strongest applause lines are when he talks about reclaiming patriotism from the Republican Party. ''I'm tired of listening to the armchair patriots in Washington who want to tell us that asking questions about the direction of our country is somehow unpatriotic,'' he said in Kansas City on Wednesday night, in remarks delivered beneath a huge American flag.

''I want to make it clear to all Americans again that that flag that's standing behind us, that flag that so many of us vets saw draped over coffins of friends, that we see flying at half staff, that flag that flew on my boat, proudly -- that flag doesn't belong to any president,'' Mr. Kerry said. ''It doesn't belong to any party. It doesn't belong to any ideology. It is the symbol of our nation. It belongs to all of us.''

He is, in short, presenting himself as a patriot with a different view, and a different biography. Mr. Bush served stateside in the Texas Air National Guard during Vietnam.

At Thursday's event in Independence, Mo., Gen. Johnnie Wilson, one of only four black four-star generals, got a standing ovation when he introduced Mr. Kerry this way: ''It seems to me unpatriotic that those who were absent would question Kerry's commitment, dedication and patriotism to our great nation.''

The Democrats' aggressiveness in this debate is, in large part, a reaction to 2002, strategists close to the campaign say. The Democrats' vulnerability on national security that year, one year after the terrorist attacks, was embodied by the Senate race in Georgia. Max Cleland, a triple amputee who earned a silver star in Vietnam, lost that seat to Saxby Chambliss after a lacerating Republican advertising campaign that used footage of Saddam Hussein and Osama bin Laden to accuse Mr. Cleland of being soft on defense.

''There's a strong feeling among Democrats that in going after Max Cleland, who left three limbs on the ground, Republicans really crossed a line,'' said a Democratic strategist close to Mr. Kerry. ''That upset John Kerry and a lot of other Democrats.''

Moreover, Democrats have a longstanding vulnerability on national security, pollsters say, which they must address in an age of terrorism. ''The biggest doubt about Democrats coming into this election was on the military -- do they respect the military and will they support spending for the military,'' said Stan Greenberg, a Democratic pollster. ''Kerry is making considerable progress on that.''

Still, Mr. Bush's image of strength in protecting the nation from terrorism remains a fundamental advantage in the polls.

Steve Schmidt, a spokesman for the Bush campaign, said his party was not attacking Mr. Kerry because he was offering criticisms, but because he was inconsistent. ''George McGovern ran as an antiwar candidate,'' Mr. Schmidt said. ''That was his position, and it was appropriate for him to do that. What John Kerry has done is utterly unprincipled.''

Where the Kerry campaign highlights the candidate's military service, the Bush campaign asserts the focus, more rightly, belongs on Mr. Kerry's 19-year voting record in the Senate, including various votes against weapon systems.

Mr. Devine, the Kerry adviser, said, ''They would like him to talk about something other than his strengths, and it's not going to happen.''

Mr. Kerry will shift his focus to domestic policy next week, when Mr. Bush is overseas meeting with foreign leaders -- an implicit acknowledgement of the old tradition in American politics, (often broken), that partisanship ends at the water's edge.

Still, the rivalry continues, more subtly. Mr. Bush will stand this Sunday before one of the most powerful and iconic scenes in American history -- the beaches of Normandy.

Mr. Kerry will address a high school graduating class from Temperance, Mich., where a senior, Brandon Spader, headed off to the Air Force Academy wrote to him last March. ''Our town has been through so much,'' Mr. Spader said. ''We have lost servicemen to a war, jobs to a depressing economy and neighboring Canada, and teachers to a cut in education spending.''

Photos: President Bush on Wednesday with the Thunderbirds, the elite flight precision team of the Air Force, in Colorado Springs. Mr. Bush will honor the 60th anniversary of the D-Day invasion on Sunday in France. (Photo by Reuters); Senator John Kerry, center, introduced Lt. Cmdr. Roy Gibson, left, to Joseph Lesniewski at the World War II Memorial opening on Saturday. (Photo by European Pressphoto Agency)