WASHINGTON — Senator Clinton is proposing new measures to guard against what she says is the “most significant” of all threats to America’s security: nuclear proliferation.

Decrying a lack of urgency in the Bush administration’s response to the threat, Mrs. Clinton yesterday announced legislation to increase funding for the protection of nuclear reactor sites worldwide and to require the president to appoint a senior White House advisor on nuclear terrorism.

She outlined the proposal in a Washington speech to the inaugural forum of the Center for New American Security, during which she singled out nuclear proliferation as paramount among a litany of failures that she attributed to President Bush’s foreign policy.

Several presidential candidates have cited nuclear proliferation as a major threat to America, most notably Mrs. Clinton’s top Democratic rival, Senator Obama of Illinois, who in April vowed to “secure all nuclear weapons and material at vulnerable sites” within four years.

An Obama campaign spokeswoman, Jennifer Psaki, yesterday highlighted a bill he authored with Senator Lugar, a Republican of Indiana, that took effect earlier this year and boosts efforts to detect nuclear material and destroy conventional weapons stockpiles.

Mrs. Clinton offered her bill yesterday not as a campaign pledge, but as a measure requiring immediate action.

“I hope we will turn to this with greater urgency now,” she said. “We cannot wait 18 months for the next administration.”

She said that while the Bush administration had “joined the chorus” in recognizing the dangers of proliferation, its efforts have fallen well short. “Unfortunately, their response has foundered at the nexus of ideology and intransigence,” she said, citing the administration’s reluctance to engage directly with Iran and North Korea.

Her legislation, titled the “Nuclear Terrorism Prevention Act of 2007,” would authorize $400 million in new spending, including $200 million in foreign aid to help Russia convert highly enriched uranium to lower levels. It would also provide $100 million for security upgrades at weapons facilities around the globe.

The money may not all come from taxpayer dollars, however. Mrs. Clinton mentioned in her speech that Warren Buffett, who appeared with her at a campaign fund-raiser on Tuesday night, had pledged $50 million to help fund nuclear reactor conversion and that he said he could raise much more if the government made a greater effort to lead on the issue.

A White House spokesman, Trey Bohn, would not comment on the specifics of Mrs. Clinton’s bill, but he said the president “believes every civilized nation has a stake in preventing the spread of weapons of mass destruction and has acted accordingly on behalf of the American people.”

In her speech, Mrs. Clinton criticized the president on a range of foreign-policy issues and repeated her call for an end to the Iraq war. She said she and Senator Byrd of West Virginia planned to attach their legislation deauthorizing the war as an amendment to the upcoming defense appropriations bill.

But as she has done during the Democratic presidential debates, she pushed for aggressive policies in fighting terrorism and a willingness to use military force to stop the killing in Darfur. While repudiating the use of torture on enemy combatants and calling for the closure of the American prison at Guantanamo Bay, Cuba, she said, “I believe there can be no mercy for those who perpetrated 9/11 and other crimes against humanity.”

Her address came at a conference launching the Center for a New American Security, a think tank whose board includes veterans of the Clinton and current Bush administrations, including a former Clinton defense secretary, William Perry, and a former undersecretary of state for Mr. Bush, Richard Armitage.