Democrats need to spell out clearly the convictions that underlie their vision of American leadership in the post-9/11 world. Fortunately, in President-elect Barack Obama they have a supremely articulate messenger who is intellectually up to the task.

Voters continueto harbor doubts about Democrats on security. One poll in September, just as the financial crisis was breaking, gave Republicans a 14-point advantage on questions of terrorism and security. It found that voters were increasingly likely to view Democrats as indecisive in facing threats and reluctant to use force; as insufficiently supportive of the military, and as following public opinion "rather than adhering to a consistent, principled view of the country's best interests."

So, among the big challenges Mr. Obama faces heading into his first term is to close the national security confidence gap. At the same time, the Bush administration's reckless disregard for settled principles of U.S. foreign policy has given Mr. Obama and the Democrats an opening to reclaim their liberal internationalist tradition and bring it into the 21st century.

In calling for a new, progressive internationalism, Mr. Obama would be preaching to a national choir. Americans may be Jacksonians when attacked, but they are also instinctive internationalists. They understand that alliances don't tie America's hands so much as extend our global reach and make our actions more legitimate in the eyes of the global public. Polls have consistently shown overwhelming support for Mr. Obama's call to rebuild America's strategic alliances. His global popularity will certainly make that job easier, and it gives him a fund of goodwill he can tap as he tries to persuade our allies to do difficult things, like staying in Afghanistan and stiffening sanctions on Iran.

And while conservatives perennially fret about eroding U.S. sovereignty, Americans for more than a half-century have embraced the wisdom of embedding U.S. power in networks of global institutions that promote global cooperation in solving common problems. In today's increasingly interdependent world, no nation is strong enough to go it alone. We need other countries' help to solve problems of the global commons like today's financial crisis, terrorism, climate change, the depletion of natural resources, pandemics and poverty.

The new internationalism must recapture the spirit of tough liberalism exemplified by Presidents Harry S. Truman and John F. Kennedy. As Kennedy famously pointed out, Cold War liberals did not fear to negotiate, but they understood that U.S. diplomacy is most effective when backed by the credible threat of force. So they kept U.S. forces strong and showed resolve in defending open societies against their enemies.

Speaking of resolve, it took Russian President Dmitry Medvedev just two days to confirm Joe Biden's famous prediction that Mr. Obama would be "tested" by foreign adversaries. Mr. Medvedev threatened to deploy short-range missiles near Poland unless Mr. Obama abandons the Bush administration's plan to deploy missile defenses there. Cool reason and quiet diplomacy may yet head off an early confrontation with Russia, but in any case President-elect Obama must stand firm against such blatant attempts at intimidation.