Critics cast doubt on the authenticity of the threat after John Ashcroft launched a hunt on Wednesday for seven "armed and dangerous suspects".
Harold Schaitberger, president of the firefighters' union, said on Thursday that the administration had known about the threats for a month.
He said: "I do find it awfully convenient and suspicious that it happens to be tied in right behind the president's recent message to the nation as well as his troubling, plummeting poll numbers."
And presidential contender John Kerry said the American people deserve a president who "makes the country safer".
He told a rally in Seattle: "We deserve a president of the United States who doesn't make homeland security a photo opportunity."
"We deserve a president of the United States who doesn't make homeland security a photo opportunity"
US presidential candidate
The comments came after Ashcroft said "credible intelligence from multiple sources indicates that al-Qaida plans to attempt an attack on the United States in the next few months".
Ashcroft and Federal Bureau of Investigation director Robert Mueller released the names of seven suspects, including one woman, who could be part of an attack plot.
"Beyond this intelligence, al-Qaida's own public statements suggests that it is almost ready to attack the United States," Ashcroft added.
The new alert came just ahead of Saturday's dedication of a second world war memorial by President George Bush which is expected to draw about 100,000 people to Washington.
Authorities fear the upcoming Republican and Democratic party conventions and the presidential election on 2 November could also be targets.
However, Ashcroft highlighted there had not been a terrorist strike on US soil since the September 11, 2001 attacks on New York and Washington that killed around 3000 people.
Irene Khan (R) says the US's war
on terror is 'bereft of principle'
"We are winning the war on terror," he asserted.
Aschcroft's alert came on the same day that a respected human rights group lambasted the US for abuses in the name of the "war on terror".
In a new report Amnesty International said Washington's global anti-terror policies are "bankrupt of vision" as human rights become sacrificed in the blind pursuit of security.
Amnesty chief Irene Khan said: "Violating rights at home, turning a blind eye to abuses abroad and using pre-emptive military force where and when it chooses has damaged justice and freedom, and made the world a more dangerous place."
Specifically, Amnesty lashed Washington for unlawful killings of Iraqi civilians; questionable arrest and mistreatment of prisoners in Iraq, Guantanamo Bay and Afghanistan; and opposition to a new global criminal court.