A bid to lift a ban on gay rabbis and same-sex unions is posing a challenge for Conservative rabbis whose U.S.-based world movement prides itself on balancing Jewish tradition with modernity. Conservative rabbis meeting in Mexico City this week say it is no easy task for a group that once dominated American Judaism but in 15 years has been overtaken by the more liberal Reform movement as the biggest in the United States. “One doesn’t easily overturn thousands of years of tradition,” Kenneth Cohen, a rabbi to students at American University in Washington, D.C., said at the meeting in Mexico. But the rabbi said that, like most of his constituents, he favored lifting the ban. “Judaism never has existed in a vacuum,” he said. “‘Halakha’ — Jewish law — is the application of Jewish values to real life.” The last time the movement considered the homosexuality issue, in 1992, it said gays and lesbians would be welcome in the movement but it barred gay behavior, ordination, membership in the Rabbinical Assembly and same-sex commitment ceremonies. A growing number of Conservative rabbis have voiced support for lifting the ban. They say homosexuals want to come out of the closet, be ordained at rabbinical schools and receive religious blessings the same as heterosexual couples.