"Candidates should feel comfortable explaining their religious convictions to voters," the league said in a statement. "At the same time, however, we believe there is a point at which an emphasis on religion in a political campaign becomes inappropriate and even unsettling in a religiously diverse society such as ours."
The statement was signed by Howard P. Berkowitz, national chairman of the league, and its director, Abraham H. Foxman. The league's focus is fighting anti-Semitism.
Lieberman, the first Jew on a major political party's ticket, told an interfaith breakfast in Chicago on Monday: "This is the most religious country in the world and sometimes we try to stifle that fact or hide it. But the profound and ultimately most important reality is that we are not only citizens of this blessed country, we are citizens of the same awesome God."
The league cited specifically Lieberman's remarks Sunday to the congregation of a church in Detroit. He said he hoped his candidacy as an Orthodox Jew would reinstate "a place for faith in America's public life."
"As a people, we need to reaffirm our faith and renew the dedication of our nation and ourselves to God and God's purposes," Lieberman said.
The Anti-Defamation League responded: "Language such as this risks alienating the American people."
"We feel very strongly, and we hope you would agree, that appealing along religious lines, or belief in God, is contrary to the American ideal," Berkowitz and Foxman said in a letter sent Monday to Lieberman, a Connecticut senator. "The First Amendment requires that government neither support one religion over another nor the religious over the nonreligious."
The organization said it sent similar letters to eight presidential candidates in December before the start of the primary election season. Vice President Al Gore, the Democratic presidential nominee, and the Republican nominee, Texas Gov. George W. Bush, have spoken often of their own religious beliefs.
Bush told the annual convention of B'nai B'rith International in Washington on Monday, "Our nation is chosen by God and commissioned by history to be a model to the world of justice and inclusion and diversity without division."