On a cool April evening, over dinner and drinks, Twin Cities Archbishop John Nienstedt gathered a group of non-Catholic clergy leaders at his St. Paul home to begin forging an alliance to persuade Minnesota voters to pass a constitutional amendment banning same-sex marriage.

"He's reached out to us and we've reached back to him," said Carl Nelson, president of Transform Minnesota, a network of nearly 160 evangelical churches in Minnesota.

Working aggressively behind the scenes, the 65-year-old Nienstedt has emerged as a key financial and political force for passage of the marriage amendment, which will be on the Nov. 6 ballot and is the most contentious issue in the state this election season.

He has committed more than $650,000 in church money, stitched together a coalition of leaders from other faiths and exerted all his power within the church to press Minnesota's million-plus Catholics to back him.

"We wouldn't have gotten very far without him," said Frank Schubert, campaign manager for Minnesota for Marriage, the lead group pushing the amendment. "What the archbishop is doing in Minnesota is what the pope asked him to do. It's hard to overstate his importance."

But Nienstedt's central role in the campaign has also brought blistering criticism from the faithful.

"I just see that this is terrible. This is not how Christ would have spent this money," said Pauline Cahalan, 67, a lifelong Catholic from Roseville.

"It's very concerning to me when someone says you have to think like I tell you to think."

Nienstedt strongly defended his stance in a written response to Star Tribune questions last week. He said he sees no problem enshrining a religious belief about marriage in the state Constitution.

"Marriage defined as a union between one man and one woman is a reality that predates any government or religious denomination," Nienstedt said. "Marriage is meant for children and children flourish best with a mother and a father."

When asked whether a loyal Catholic could vote against the amendment, Nienstedt said: "It would be difficult to comprehend how a person could not believe that marriage is anything but a union between one man and one woman. On this point, Catholic teaching is clear."

The Vatican is keeping a close eye on the outcome of the marriage amendment vote. In March, Pope Benedict told Minnesota bishops visiting in Rome that the preservation of traditional marriage must be a top priority. Bishops who fought to uphold the definition of marriage in other states have been rewarded with promotions. For an ambitious archbishop, the marriage amendment offers the potential for advancement in the Catholic hierarchy.

The political fight has pushed Nienstedt onto the campaign trail. Last month he joined nearly 40 evangelical and other leaders on the steps of the State Capitol to make a public appeal for the amendment.

"I explain and defend the teaching of the Church because I have been ordained to do so and I believe those teachings with all of my heart," he said.

Clear priority for years

Nienstedt is not a new disciple to the traditional marriage campaign. In 2006, as bishop of the diocese of New Ulm, he mobilized Catholics to send postcards to lawmakers urging them to support a constitutional ban on same-sex marriage.

Not long after he was promoted to archbishop in 2008, Nienstedt ordered an end to the gay pride prayer service at St. Joan of Arc Church in Minneapolis. Before the 2010 election, he led a move to send DVDs opposing same-sex marriage to 400,000 Catholics in Minnesota, in which he gave a six-minute introduction.

Jason Adkins, executive director for the Minnesota Catholic Conference, the public policy voice of the Catholic Church, said Nienstedt's campaign sprang from a 2003 U.S. Supreme Court ruling that legalized same-sex sexual activity and "gave tremendous impetus to the movement to redefine marriage throughout the country."

Schubert, a longtime political strategist who has won marriage-related campaigns in California and other states, said he first met Nienstedt in 2010. Same-sex marriage opponents were worried at the time that the DFL-controlled Legislature was going to take a run at the state's marriage laws. Activists contacted the National Organization for Marriage, a group Schubert was working with. The group reached out to Nienstedt and found an ally.

Nienstedt worked with Schubert to produce the DVDs and to put his views in writing. In a December letter to clergy, Nienstedt wrote that "the institution of marriage and family life are unraveling before our very eyes due to no-fault divorce, widespread cohabitation and promiscuous sexual activity." Amendment opponents, he said, seek "to eliminate the need for marriage altogether."

Many Catholics have welcomed his activism.