In remarks prepared for delivery Wednesday to the National Press Club, Kennedy, D-Mass., said Democrats must do a better job speaking about the principles they believe in and that have guided the party.
"We cannot move our party or our nation forward under pale colors and timid voices," said Kennedy, who has served 42 years in the Senate. "We cannot become Republican clones. If we do, we will lose again, and deserve to lose."
But at the same time, Kennedy said Democratic Sen. John Kerry's narrow election loss also showed that the party must "recognize issues of deep conscience in policy positions we take." Referring to abortion, he said Democrats should not yield on a woman's right to choose, but should also acknowledge that "we are a better society when abortions are rare."
Kennedy's speech came as Democrats — divided and battered by the second bitter presidential defeat in a row — continue to wrangle over their party's direction.
Ever since Kerry's loss — and GOP's gains in both the House and Senate — Democrats have been chewing over their inability to connect with enough voters to wrest the Oval Office from a president weakened by a faltering economy and increasingly unpopular war in Iraq.
And they have debated how to compete with Republicans for the support of social conservatives whose votes may have been swayed by hot button family values issues like abortion, religion and gay marriage.
Some pundits have called for the party to get back to its liberal roots and take back the moral high ground, where the GOP has successfully gained traction. But others have recalled Bill Clinton's success in taking a centrist approach to the White House.
On Wednesday, Kennedy laid down markers for the coming congressional session, vowing to defeat President Bush's efforts to revamp Social Security, and reject policies that send jobs overseas. And he ratcheted up his assault against the administration's handling of the war and its aftermath in Iraq.
Raising the specter of Iraq as Mr. Bush's Vietnam, Kennedy said the administration has bogged America down "in an endless quagmire." And in a brief glance backward, he said Kerry could have worked better with the international community to end the war and bring the troops home.