"When you have uncertainty in the area of nuclear weapons, that's a much more dangerous world to live in," Obama said from the Oval Office after a meeting with Powell and Vice President Joe Biden.
Powell, a retired four-star Army general and former Joint Chiefs of Staff chairman, said he fully supports the new START treaty, and believes Obama has adequately addressed concerns of Republicans lawmakers over verification systems and modernization of the remaining U.S. nuclear arsenal.
Failing to ratify the treaty, Powell said, could leave the U.S. in a vulnerable position.
"We're not exactly sure what's going on in the Russian Federation, and they're not exactly sure what's going on in the United States," said Powell, who joined Obama in urging the Senate to ratify the treaty by the end of the year.
White House officials were cautiously optimistic Wednesday that momentum was building toward the treaty's ultimate ratification. Officials specifically pointed to comments this week from Sen. George Voinovich, R-Ohio, and Sen. John McCain, R-Ariz., who both indicated they'd like to finish work on the treaty this year.
Leading Republican senators have begun to argue that any action on START would have to come after the Senate addresses an extension of Bush-era tax rates and legislation to keep the government operating during the lame-duck session.
Republicans have threatened to block any other legislative action that reaches the floor of the Senate. But that threat, spelled out in a letter to Senate Majority Leader Harry Reid, specifically did not apply to the START treaty.
"Were we to finish the two things that we absolutely must do - taxes and spending - the more time that would be left for something else," Senate Republican leader Mitch McConnell of Kentucky said.
Reid also wants to hold votes on immigration legislation and ending the military's don't-ask, don't-tell policy for gay service members.
START would reduce how many strategic warheads the United States and Russia could hold and set up a system so each could inspect and verify the other's arsenal.
The White House has sought to cast the treaty's ratification as a national security imperative, not a political issue. Officials have underscored the need for the U.S. to show credibility in its newly improved relationship with Russia, whose support is vital in providing supply help for the war in Afghanistan and strengthening international pressure on Iran over its nuclear intentions.
"The relationships and trust that have built from the new START treaty spill over into a whole host of other national security issues that are of vital importance to America," Obama said Wednesday.
Russian Prime Minister Vladimir Putin warned that his country would build up its nuclear forces if the U.S. doesn't ratify the treaty.
In an interview to be broadcast Wednesday on CNN, Putin said that if the treaty isn't ratified, "we'll have to react somehow," including deploying new nuclear technology. Putin said it would be "very dumb" for lawmakers to block the treaty.
Powell is the latest in a string of current and former national security officials the Obama administration has brought in to bolster the need to ratify the treaty.
Last month, Obama met at the White House with what he called "some of the most able statesmen from both parties" to get support for START. They included former secretaries of state Madeleine Albright, James Baker III and Henry Kissinger, former defense secretaries William Perry and William Cohen and former National Security Adviser Brent Scowcroft.
During a NATO summit in Portugal, Obama also secured the backing of several Eastern European nations, easing Voinovich's concerns that the treaty would undermine the confidence of allies in Central and Eastern Europe.
One top Republican said Wednesday that Powell's endorsement was unlikely to sway him to vote for the treaty.
"I respect Secretary Powell a lot, but this is a matter of us doing our job, doing our due diligence," Sen. John Cornyn, R-Texas, said.
McCain, whose vote on the treaty would be critical for the White House, said Powell's endorsement didn't come as a surprise.
Powell, who served as secretary of state under former President George W. Bush, endorsed Obama in the presidential election over McCain.