President Bush, meanwhile, is going to Las Vegas, where he will deliver a speech to a National Guard convention.
Some of the 9/11 widows, including Kristen Breitweiser, of Middletown, N.J., and Monica Gabrielle, of West Haven, Conn., also have agreed to make campaign appearances for the Massachusetts senator.
"We will be speaking from the heart, and speaking from our conscience," Breitweiser said. She would not elaborate. Breitweiser is by far the most visible and outspoken of the Sept. 11 family advocates, and has been highly critical of the government's reform efforts to date.
The move highlights the widening political divide among the nearly 3,000 Sept. 11 families.
At the Republican National Convention two weeks ago, two widows and the sister of another Sept. 11 victim offered moving tributes to their departed loved ones. The somber appearances offered no direct endorsement of Mr. Bush, but their message of support was unmistakable.
In Las Vegas, the president is expected to honor the sacrifice of National Guardsmen in Iraq and Afghanistan, noting their special role in the war on terror, and saying he also is proud of his own Vietnam-era stint as a citizen soldier.
Mr. Bush was not expected to address the controversy swirling around unexplained gaps in his service in the Texas Air National Guard in his address to the National Guard Association of the United States.
Mr. Bush is speaking at the National Guard conference in the afternoon following a morning rally in Arapahoe County, Colo., to promote his health care agenda.
The battle against terrorism has been the rallying cry of Mr. Bush's race for re-election in a presidential campaign fraught with verbal fisticuffs between him and Mr. Kerry, who addresses the group on Thursday.
New documents disclosed last week suggest Mr. Bush ignored a direct order from a superior officer to take a physical exam and lost his status as a Texas Air National Guard pilot in the 1970s because he failed to meet military performance standards and did not get the physical.
Questions have been raised about the authenticity of the documents obtained by an unidentified source by CBS News. The news organization is standing by its story.
On Monday, first lady Laura Bush said she thought the documents were probably fakes. "You know, they probably are altered and they probably are forgeries, and I think that's terrible, really," she told Radio Iowa in an interview.
Hours before Mr. Bush addresses the convention, members of Military Families Speak Out, a group representing 1,700 military families, were to tell stories about relatives who have served in Iraq or have been killed in the conflict.
At a news conference in a hotel next to the convention center where Mr. Bush is speaking, the relatives were expected to accuse the Bush administration of betraying National Guard soldiers who signed up to serve at home, but were sent overseas for extended periods of time to fight a "war based on lies."