WASHINGTON — Hours after the Supreme Court struck down a key section of the Defense of Marriage Act, Udi Ofer of the American Civil Liberties Union spent his evening in a Unitarian church in Summit, N.J., training nearly 40 members how to persuade three local Republican lawmakers to endorse same-sex marriage.
Less than two days later, leading Christian conservative Ralph Reed was on a conference call with nine of his top donors and allied organizations discussing how to mobilize the money and resources to fight gay marriage activists not only in New Jersey, but in states ranging from Iowa to Oregon.
Gay rights activists are pushing ahead with a well-financed, coordinated campaign that aims to legalize same-sex marriage in about a dozen key states within three years. But they face fierce resistance from conservative groups and their allies in state legislatures and Congress, who hope to stymie any momentum coming out of recent court rulings.
This pitched political battle — which has already cost each side millions of dollars and is poised to escalate further — will help determine how broadly same-sex marriage is adopted over the coming decade or longer. It is also likely to play a major role in state and national elections in the near-term, as activists on both sides fight to win over the Republican voters and elected officials who are key to deciding the fight.
In two landmark decisions last month, the Supreme Court cleared the way for federal recognition of legally married gay couples and gave an opening to California that allowed it to resume such unions there.
‘‘Even before that moment, we were at work mapping the path forward toward expanding marriage equality,’’ said Evan Wolfson, founder and president of Freedom to Marry.
But Reed, who heads the Faith and Freedom Coalition, said gay marriage activists are overstating their political advantage, given that 29 states define marriage as only between a man and a woman.
‘‘They’ve been highly successful,’’ Tina Fetner, a sociology professor at McMaster University, said of the conservative movement. ‘‘What’s happened is highly historic but lesbian and gay activists aren’t winning everything they fight for, in fact it’s probably the other way around.’’
Both sides agree that a half-dozen states will be pivotal over the next three years, with several more potentially in play after that. Hawaii, Illinois, Nevada, New Jersey, New Mexico, and Oregon rank as the top near-term targets for gay activists, while opponents of same-sex unions are hoping to gain ground in Indiana and Iowa.
The campaigns vary depending on the state.
Two of the most intense battlegrounds right now are New Jersey and Illinois, where gay marriage proponents hope to eke out legislative or court victories by the end of the year.
New Jersey has allowed civil unions since a 2006 state Supreme Court equal rights decision and the legislature approved a gay marriage bill last year. But Governor Chris Christie, a Republican, vetoed the bill and has shown no signs of changing his position.
In Illinois, the state House adjourned several weeks ago without taking up a marriage bill that had passed the state Senate. The state legalized civil unions in 2011.
Conservatives will also push for a federal constitutional amendment banning gay marriage sponsored by Representative Tim Huelskamp, a Kansas Republican, although its chances are slim to none in the Democratic-run Senate.