Sheehan, a Californian, officially announced that she intends to run as an independent against Pelosi in 2008 if the San Francisco congresswoman doesn't move to impeach Bush by July 23, the day she expects to reach Washington.
"I know what Californians care about," Sheehan said. "They don't care about the ruling power elite."
Sheehan first told The Associated Press on Sunday about her plans to challenge the top-ranking Democrat. She made it official Tuesday at Camp Casey, named after her 24-year-old son, whose death in Iraq first led Sheehan to set up camp in Crawford in 2005 to demand a meeting with Bush, who was on vacation at the time.
Pelosi spokesman Brendan Daly said Sunday that "July will be a month of action in Congress to end the war, including a vote to redeploy our troops by next spring."
But Sheehan said that's not enough.
"You can't bring the troops home if you give George Bush $100 billion to wage this war," she said Tuesday. "You're not supporting them. You're keeping them in harm's way."
Sheehan says Bush should be impeached because she believes he misled the public about the reasons for going to war, violated the Geneva Convention with the torture of detainees and crossed the line by commuting the prison sentence of former vice presidential aide I. Lewis "Scooter" Libby.
After her announcement, Sheehan and about 20 anti-war protesters began their 13-day trip with a symbolic march to the edge of Crawford, where a billboard of Bush and first lady Laura Bush welcomes visitors.
Sheehan, who turned 50 on Tuesday, stunned fellow anti-war activists in May by announcing that she would sell her 5-acre Crawford protest site. She said then that she felt her efforts had been in vain and that she had endured hatred and smear tactics from the left as well as from the right.