Tonight's debate between President Bush and Sen. John F. Kerry marks the culmination of months of campaigning, speeches and attack ads, and the clash is likely to set the tone for the remaining five weeks of the election.

Each candidate has studied briefing books and held hours of practice sessions, honing the sound bites that will best capture the attention of the millions of television viewers. For many viewers, the information will seem new, but both men have been road-testing their claims as they have barnstormed the nation, in the political equivalent of an out-of-town tryout for a Broadway show.

Many of those assertions, all carefully constructed to present the candidates' cases in the best possible light, are based on a study or report discovered by teams of campaign researchers. At first glance, a candidate's assertion may have the ring of truth. But on close examination, many of their pronouncements turn out to be exaggerated, lacking in context or wrong.

That is particularly so when each candidate is attacking his opponent's stance, such as when Bush has said Kerry would "nationalize" health care, or when Kerry has said the Iraq war has already cost $200 billion.

Both men have been highly selective when it comes to using statistics on jobs. Bush cites only the jobs created in the past year -- ignoring the overall job loss in his term -- while Kerry makes the job loss seem larger by focusing on just one segment of the economy.

Tonight's debate is supposed to focus on foreign policy, but domestic issues are likely to be raised as well, and will be the subject of later debates. Here are some of the inaccurate claims Bush and Kerry have been making on crucial issues in this election year.

Foreign Policy

The war in Iraq is likely to figure prominently in tonight's debate. The two candidates differ greatly over how the president led the nation into war, and whether Bush has handled the post-invasion diplomacy effectively. But both candidates, in essence, offer simplified and often misleading accounts.

Bush emphasizes his efforts to avert war. "I went to the United Nations, because I was hoping that diplomacy would work," he tells audiences, adding that "it is documented" that Iraqi leader Saddam Hussein "systematically deceived the inspectors the United Nations sent in."

In contrast, Kerry charges: "The administration misled America, the United Nations and the world. This administration rushed to war without a plan to win the peace."

While Bush did seek U.N. approval to confront Hussein -- after sharp debate among his foreign policy advisers -- the White House very quickly gave up on the inspection process and assumed a war footing several months before the March 2003 invasion, according to administration officials. The administration rejected several compromise proposals from other nations that would have delayed an invasion and allowed inspectors to continue searching for weapons of mass destruction. No such weapons were found after the invasion, and much of the prewar intelligence the administration used to justify the invasion was later found to be wrong.

Investigations have found that many nuances or caveats about the intelligence were ignored or belittled in assembling reports on Iraq's alleged weapons. But there is little evidence the Bush administration purposely tried to deceive Americans and other world leaders about the threat posed by the alleged weapons -- and, in fact, there was a broad consensus among experts that Iraq did have at least some banned weapons.

Many of the assumptions the administration had about rebuilding Iraq after the war did turn out to be incorrect, and the administration's post-invasion plans have been revised repeatedly.

Bush has charged that Kerry has said "he prefers the stability of a dictatorship to the hope and security of democracy." But Kerry has never said that. The Bush campaign said this attack is derived from a Kerry statement that "the satisfaction that we take in [Hussein's] downfall does not hide this fact: We have traded a dictator for a chaos that has left America less secure." Kerry prefaced that statement, however, by saying that while Hussein was "a brutal dictator who deserved his own special place in hell," that by itself was not a reason to go to war.

In a recent line of attack, Kerry has said the cost of Bush's "go-it-alone policy in Iraq is now $200 billion." This is an exaggeration, because it combines the amount already spent -- about $120 billion -- with money that is expected to be spent in the coming year or requested by the administration. In the past week, Kerry has modified his comments to say Bush "didn't tell America this would cost $200 billion."

Economic Policy

Bush and Kerry offer a blizzard of statistics about the economy, many carefully selected to make things look as good or bad as possible.

Bush, for instance, is fond of saying, "We've added 1.7 million jobs since August 2003" -- ignoring the fact that the total job loss since Bush took office is nearly 1 million, making him the first president since Herbert Hoover to have presided over a net loss in jobs.

Kerry, meanwhile, emphasizes the loss of 2.7 million manufacturing jobs since Bush took office, which overstates the total job loss and ignores the historical trend toward service-oriented jobs.

Bush likes to cite the fact that the current unemployment rate, 5.4 percent, is "lower than the average of the 1970s, the 1980s and the 1990s." Though this is factually correct, comparing a one-month figure with 10-year averages is an apples-and-oranges exercise. The unemployment rate was 4 percent when Bush took office, though it is down from a high of 6.3 percent in June 2003.

Kerry lashes Bush for "settling for jobs that pay us $9,000 less than the jobs that are going overseas" -- an extreme extrapolation of figures contained in a study published by a labor-backed progressive group. Kerry also blames Bush for giving tax preferences to offshore companies, but the provision was enacted long before Bush was elected.

Kerry is on firmer ground when decrying the nation's "shrinking middle class" and the largest budget deficit in history -- though as a percentage of gross domestic product it is second to Ronald Reagan's. The latest U.S. Census Bureau data show median household income was flat or declined during Bush's tenure and the number of Americans living in poverty rose by 4 million, or 14 percent.

One of Bush's most effective lines on the stump is that he is running "against a fellow who's promised over 2.2 trillion new dollars of federal spending so far." The Kerry campaign disputes that estimate -- and Bush's spending proposals top $3 trillion, according to administration figures.

Health Policy

The differences between the candidates are especially profound on health policy.

Bush brags frequently about last year's passage of Medicare prescription drug coverage. "I believe we have a moral obligation to provide our seniors with good health care," he says on the trail. "We've strengthened Medicare, and we're not turning back."

But the estimated cost has risen from $400 billion over the next 10 years to $576 billion; independent investigators have excoriated administration officials for concealing the true costs. Retirees complain that coverage does not begin until 2006 and that the interim drug discount cards are confusing. Bush says 4 million seniors have signed up for the cards -- but nearly 3 million were enrolled automatically, and the number enrolled is 3.3 million below the administration's target.

When Bush says he "strengthened" Medicare, he is referring largely to new preventive services, such as a "welcome to Medicare" physical for every 65-year-old. Medicare's independent trustees, however, report that the law severely weakened the program's fiscal stability.

Kerry taps into seniors' frustration by touting his opposition to the bill, though he missed the vote on final passage. His "no" vote would not have changed the outcome, however, and he was on the Senate floor for a crucial procedural vote that would have killed the legislation. He describes the law as a "$139 billion payoff" to drug companies. The charge is based on a Boston University study that some experts say undercounts the potential savings that come with Bush's new cost controls.

Each man blames the other for the recently announced 17 percent jump in 2005 premiums -- and neither is being fair. When Bush accuses Kerry of voting five times to raise premiums, he's counting votes on spending bills that, among other things, retained a formula setting premiums at one-quarter the total cost. In all but one instance, the votes were bipartisan.

Similarly, Bush cannot be blamed for the formula or rising health costs overall. But the Centers for Medicare and Medicaid Services, a federal agency, estimated that 9.9 percent of the increase was due to changes in the law Bush signed, namely higher payments to physicians and managed care companies.

Bush raises the specter of Hillary Rodham Clinton's failed health care reform with comments that Kerry would "nationalize" health care. Bush says Kerry has crafted a plan "that is massive, it is complicated, it is a blueprint to have the government control your health care," and he mocks his rival for plans to expand Medicaid, the federal-state program for the poor and disabled.

While Kerry aims to spend a good deal of federal money ($653 billion to $1.5 trillion over 10 years) to extend coverage to about 27 million of the 45 million uninsured Americans, he would not "nationalize" health care. Kerry would build on the existing health system, expanding some government programs and broadening access to private plans.

Bush would spend considerably less ($93 billion to $130 billion over a decade) to cover 2 million to 10 million uninsured Americans. And his approach, many experts say, would mean much more upheaval in the current system, moving away from employer-based insurance to individual coverage.

Researcher Lucy Shackelford contributed to this report.

Technicians at the University of Miami prepare the stage for the initial presidential debate.