LAS VEGAS (Reuters) - President Bush, accused by Democrats of joining the National Guard to avoid going to Vietnam and then shirking his duty, told a Guard convention on Tuesday he is proud of his service but did not address lingering questions about his record.

Bush then took aim at Democratic presidential rival John Kerry, a decorated Vietnam veteran. Challenging his fitness to lead, Bush accused Kerry of wavering on funding for the war in Iraq and warned against choosing a commander-in-chief who would shift positions "because of expediency or pressure" at a time of war.

With Guard members facing extended deployments in Iraq and Afghanistan, Bush promised more generous benefits and said he would try to give them more time to prepare for future mobilizations.

Bush did not directly address the controversy surrounding gaps in his service in the Texas Air National Guard in the early 1970s.

But he said: "Nineteen individuals have served both in the National Guard and as president of the United States, and I am proud to be one of them."

That brought a roar of cheers followed by a standing ovation from the National Guard crowd.

White House spokesman Scott McClellan said Bush did not address the underlying charges by Democrats because they had already been addressed. "He met his obligations and he was honorably discharged," McClellan said.

Critics say extended deployments in Iraq and Afghanistan have stretched U.S. forces too thin, forcing Bush to rely more heavily than ever on the part-time National Guard.

A group of National Guard families planned to protest the war in Iraq during Bush's address in Las Vegas.

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

He said his administration hoped to provide Guard members with at least 30 days notice for future mobilizations and provide "as much certainty as possible" about the length of mobilizations, and to minimize the number of extensions and repeat mobilizations.

'OUR ENEMIES WILL NOT WAVER'

In Las Vegas and at an earlier rally in the swing state of Colorado, Bush sought to undercut Kerry's standing with fellow veterans, a key voting bloc in the November election.

He criticized Kerry for voting against funding for operations in Iraq, when last year "he had a completely different view" and advocated an increase in funding.

"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," Bush said. "We cannot waver because our enemies will not waver."

With national security issues figuring prominently in the campaign, the military records of both Bush and Kerry have come under intense scrutiny.

Kerry has been pummeled for weeks by Republican-backed charges that he lied about his war record in Vietnam. Kerry was slow to respond to the assault, which contributed to his slide in the polls.

Democrats have sought to turn the tables on Bush in recent days by pointing to gaps in his Guard service and a CBS report on memos from a deceased Guard officer who questioned whether Bush had received preferential treatment.

The new memos, whose authenticity have been questioned, also assert Bush lost his status as a pilot because he failed to meet performance standards.

First lady Laura Bush, in an interview with Radio Iowa, said the documents in question were "probably" forgeries. "I think that's terrible, really," she said. She did not explain why she thought they were forgeries.

Since the Sept. 11, 2001 attacks, more than 225,000 Guard members have been activated for full-time duty. Nearly 40 percent of the U.S. troops deployed for service in Iraq and Afghanistan are from the National Guard and Reserve.