President Bush ignored a fresh round of questions about his service in the Texas Air National Guard and, instead, repeatedly questioned the character of challenger John F. Kerry during an appearance Tuesday before a National Guard convention here.

Pointing to the Democrat's evolving position on how much the United States should be spending in Iraq, Bush said: "What's critical is that the president of the United States speak clearly and consistently at this time of great threat in our world, and not change positions because of expediency or pressure."

"Our troops, our friends and allies, and our enemies must know where America stands -- and that America will stand firm," Bush said to a cheering audience of 4,300 active and retired Guard members at the 126th national conference of the National Guard Association of the United States. "We cannot waver, because our enemies will not waver."

Bush was referring to a speech last week in which Kerry said the more than $144 billion the United States has spent on the Iraq conflict -- the senator put the figure at $200 billion -- has hurt jobs, health care and education at home. He repeated his criticism that Kerry had joined most Senate members in giving him authority in 2002 to wage war against Iraq but later voted against money for the war.

Kerry is to speak to the convention on Thursday. A group called Military Families Speak Out organized a protest next to the convention center, with several families of Guard soldiers making statements opposing what the group's statement called "a war based on lies."

Although Bush holds a commanding lead over Kerry among veterans, according to polls, Tuesday's appearance was crucial for the president because of public concern about the rising death toll of U.S. troops in Iraq and renewed questions about Bush's National Guard service during the Vietnam War.

Bush had gaps in his attendance record with the Texas Air National Guard from 1969 to 1972 that have provoked periodic attacks in campaigns, and available records show a highly motivated and rated fighter pilot whose participation later became sporadic. CBS News's "60 Minutes" reopened the long-running controversy over his service last week by showing documents purporting to prove that Bush had defied a direct order to take a medical examination required for remaining a qualified pilot. The authenticity of the documents was quickly questioned, and first lady Laura Bush said on Monday that they are apparent forgeries.

Bush said nothing about the brouhaha but noted that 19 presidents have served in the National Guard. "And I am proud to be one of them," he added. White House press secretary Scott McClellan said that Bush voters are concerned about the future, not about such questions, and that the bottom line is that Bush was honorably discharged.

The Democratic National Committee on Tuesday announced "Operation Fortunate Son," an effort to impugn Bush's credibility by drawing attention to questions about his Guard service, with 30 events planned in 21 states this week. A video that the committee is distributing widely uses clips of various statements Bush has made about his service, paired with apparently contradictory testimony from others. Republicans said all legitimate questions have been answered.

Meanwhile, Vice President Cheney stepped up his attacks on Kerry's shifting positions on the war in Iraq as he campaigned in a swing area in West Virginia and a Democratic corner of Arkansas.

Appearing in a warehouse in Blytheville, Ark., where cotton is compressed into bales, Cheney chided Kerry in taking issue with presidential contender Howard Dean's opposition to the war during the Democratic primaries nine months ago.

"Senator Kerry said [then], 'Those who doubted whether Iraq or the world would be better off without Saddam Hussein and those who believe today that we are not safer with his capture don't have the judgment to be president or the credibility to be elected president,' " Cheney told a crowd of 300 supporters. "That's a direct quote from John Kerry on December 16 of last year."

For the second day in a row, Bush did not specifically mention the burgeoning bloodshed in Iraq, including Monday's car bombing in Baghdad and two separate attacks on Iraqi police and police recruits on Tuesday. "Despite ongoing acts of violence, Iraq now has a strong prime minister, a national council, and national elections are scheduled for January," he said.

Kerry issued a statement calling Bush's comments distortions: "George W. Bush keeps saying that things are getting better even when we all know that's just not true."

About 52,000 Guard troops are serving in Iraq and Afghanistan -- roughly a third of the U.S. forces there -- and more than 100 have died, according to the military's National Guard Bureau. About 225,000 of the Guard's 456,000 members have been mobilized since the Sept. 11, 2001, terrorist attacks.

"I know this time of call-ups and alerts and mobilizations and deployments has been difficult for Guard members and their families and employers," Bush said. "And when our nation must call on you, we owe you some things in return."

Bush said the government is working to provide at least 30 days' notification for mobilizations, and to "give you as much certainty as possible about the length of your mobilization -- you deserve to know when you can expect to resume civilian life."

Staff writer Lisa Rein in Blytheville, Ark., contributed to this report.

At the annual meeting of the National Guard Association, in Las Vegas, President Bush listens to applause led by Col. Al Faber, left, and Maj. Gen. Gus Hargett. He declared: "We cannot waver, because our enemies won't."