Sheehan was joined by about 30 supporters in her march down Pennsylvania Avenue on Wednesday to deliver a letter to Bush urging him to pull the troops out of Iraq.
"We want to hold this administration accountable. Nobody's asking them the hard questions but you know what, we're willing to come here," said Sheehan, the California mother whose son Casey was killed in combat in Iraq.
"We're willing to spend weeks out of our lives and ask them the hard questions," she added after handing a poster-sized petition through the iron gates of the White House to a staff member who promised to deliver it.
Camping out during much of August while Bush was vacationing in Crawford, Texas, Sheehan's rallies drew crowds that sometimes numbered in the hundreds as she demanded a meeting with Bush. Bush, who met with Sheehan in 2004 after her son was killed, refused to meet with her a second time.
Organisers of Saturday's anti-war rally on the Ellipse behind the White House say they expect tens of thousands of people to turn out against the Iraq war.
The fledgling movement appeared to stall after the protesters left Bush's ranch, largely overshadowed by the Hurricane Katrina catastrophe.
Though a second powerful hurricane now threatens the Gulf coast, the protesters said the fighting in Iraq didn't stop because of hurricanes or other bad news.
"There's still a war on," Sheehan said on Capitol Hill. "Nine Americans were killed yesterday in Iraq. We will end this war. We will bring the troops home."
"We want to hold this administration accountable. Nobody's asking them the hard questions but you know what, we're willing to come here"
Sheehan and about 30 fellow anti-Iraq war protesters left Texas on 31 August and split into three groups to take their message across the country.
They regrouped on Wednesday in Washington, where they will create another "Camp Casey" on the National Mall, meet with members of the US Congress, and take part in the protest march and rally on Saturday.
Most of the people travelling with Sheehan were either veterans themselves or relatives of young men and women in Iraq.