Edward Kennedy said on Tuesday that the last four years of nuclear policy under Bush had been "a constant flirtation with nuclear disaster".
The Massachusetts Democrat said Bush had rejected a "half century of success" in nuclear deterrence and steps toward disarmament.
In a speech to the Carnegie Endowment for International Peace, Kennedy also faulted the president for "encouraging new arms races, neglecting arms control and ignoring the truly threatening nuclear weapons".
He added the administration's focus on Iraq - where no weapons of mass destruction have been found - and its unwillingness to work with other countries has been a serious setback for non-proliferation.
New arms race
The senator criticised the administration in particular for doing nothing to stop Pakistani scientists from selling nuclear secrets.
He said it sent a message to the world that "if you're a friend, you will not be punished for trading in nuclear arms. If you're Iraq, we will punish you, whether you really have nuclear arms or not."
Kennedy also said Bush was undermining efforts to control weapons by pursuing new categories of weapons - low-yield "mini-nukes" and earth-penetrating "bunker-busters".
"We are trying to persuade the world to 'do as we say, not as we do,' and few countries will oblige," he said.
"There is little doubt that these new categories of weapons would spark a new arms race."
Low poll numbers
The Kennedy speech came on the heels of a new poll which showed public confidence in Bush's ability to fight terrorism at a new low.
The ABC News/Washington Post poll said that approval of Bush's handling of the US campaign against terrorism had fallen to 50%, down 8 points in the last month and 29 points below its post Iraq-war peak.
"Bush ... has weakened in his once-strongest area," ABC said in reporting the poll on its Web site.
In addition, Americans now rate Democratic presidential
candidate Senator John Kerry level with Bush in ability to combat terrorism. Bush had led Kerry by 13% on the issue a month ago, and by 21 points the month before, but the new poll showed Kerry with 48 points to Bush's 47 points.
However, the poll also found that Americans continued to
pick Bush over Kerry as someone who could make the country more secure.