By Anthony Boadle

BRASILIA (Reuters) - Brazil's increasingly powerful evangelical Christians are tantalizingly close to electing one of their own as president next month in what would be a historic shift for the world's largest Catholic nation.

Marina Silva, an environmentalist running neck and neck in polls with incumbent President Dilma Rousseff, is a Pentecostal Christian who often invokes God on the campaign trail and has said she sometimes consults the Bible for inspiration when making important political decisions.

Some 65 percent of Brazil's 200 million people are Roman Catholics but evangelicals are rapidly gaining followers and power.

They grew from 5 percent of the population in 1970 to more than 22 percent in 2010 and the trend has continued. Evangelical groups have made particular inroads among urban working Brazilians who benefited from economic prosperity over the last two decades and are now demanding a greater say in politics.

Recent polls show evangelical voters would support Silva over Rousseff by a margin of about 54 percent to 38 percent if the two face each other in a runoff on Oct. 26, as most expect.

In a tight race, that could swing the result.

The evangelicals' rise has drawn comparisons to the "religious right" that began to influence U.S. politics in the 1980s.

There are important differences - most Brazilians are politically well to the left of Americans, perhaps inevitably in a country with one of the world's biggest gaps between rich and poor. Silva and Rousseff both call themselves socialists and push for robust welfare programs.

Infighting within evangelical groups has also limited their ability to create a unified bloc.

Yet similarities with the "religious right" abound. Brazil's evangelical faithful have turned their opposition to gay marriage and abortion, which are both illegal here, into key national political issues.

Funded by the tithes their followers are asked to pay, the more successful evangelical churches are increasingly turning their newfound wealth into political influence.

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They have bought up radio and television stations across Brazil and financed campaigns to elect evangelical candidates, including many pastors, to seats in Congress.

The evangelical caucus in Congress showed its muscle in May by forcing Rousseff to revoke authorization for public health service abortions in exceptional cases of pregnancies caused by rape and of fetuses with brain defects.

For the first time in a Brazilian election, there are two evangelical candidates running for president. Silva has eclipsed the second hopeful, Pastor Everaldo, although he has made his mark in debates by accusing Rousseff's government of trampling on family values and seeking to legalize abortion.

Under evangelical pressure, Silva has changed her party's position on gay rights. And Rousseff, a Catholic who has rarely used faith in her political career, is now presenting herself as a good Christian. "Happy is the nation whose God is the Lord," she quoted from Psalms at one campaign stop.

"The evangelical vote will be decisive in this election," said Rodrigo Delmasso, a pastor of a Brasilia-based Pentecostal church, who is running for a seat in the city's legislature.

"As the community grows it's natural that our share of political representation grows too," the 34-year-old pastor said during a campaign stop where he handed out bumper stickers and posters to metro workers.

Delmasso said he voted for Rousseff in 2010, but he now backs Silva, trusting she will sweep out corruption after 12 years of rule by the leftist Workers' Party.

MORALITY

Many evangelicals believe their churches are uniquely equipped to cleanse politics - and society at large.

In a shabby shopping center in the center of Brazil's capital, two theaters that used to show porn films are now churches. In one that doubled as a strip club, pastors preach about salvation from a stage where strippers once performed.

Pentecostalism, the fastest growing branch of evangelical Christianity, was introduced to Latin America by U.S. missionaries a century ago.

These days, virtually every town and neighborhood in Brazil seems to have a Pentecostal chapel where vibrant song and prayer blare over loudspeakers onto the street outside. Many converts say the uplifting services and emphasis on material prosperity are more appealing than what they found in the Catholic Church.

The exodus is happening across Latin America and is especially strong in Brazil.

The Catholic Church retains political influence of its own - gathering all the major presidential candidates for a debate in Aparecida, the shrine of Brazil's patron saint, earlier this month, for example.

But while it bans its priests from engaging in politics, evangelical preachers are free to launch their own political careers from the pulpit or televangelist studios.

Recently, at the doors of a church in Brasilia, youths in shirts and ties handed out campaign flyers to people attending the service. A vote for the evangelical congressional candidate was "the same as voting for the Church," one of the youths said.

One major goal of evangelicals is to keep expanding their caucus in Congress, which has grown from 17 in 1985 to 76 today - about 15 percent of the Chamber of Deputies.

They have in many ways followed the path taken by the U.S. Christian right, which didn't really get involved in national politics until the early 1980s when one of its own - Pat Robertson - ran for president, said Andrew Chesnut, an expert on Brazil's Pentecostal boom.

"Up until then, American evangelicals saw the political arena as the devil's arena where even forthright Christians could be contaminated," said Chesnut, a professor of religious studies at Virginia Commonwealth University in Richmond.

"The view was that if they had critical mass they could elect their own people and that would inoculate them from the contagion of the political arena. That was the same in Brazil."

CONVERSION

Silva's rise could further boost political engagement among Brazilian evangelicals. Many, like Silva, are Afro-Brazilian women who come from a poor background.

Silva came to the faith somewhat late in life. She was born to illiterate rubber-tappers in the Amazon rainforest and wanted to become a Catholic nun as an adolescent, before turning to environmental activism and a career in politics.

Her conversion to Pentecostalism came in 1997 after her doctor said only a "miracle" would heal her fragile health, wrecked by malaria, hepatitis and lead poisoning when she was a child on the rubber plantation.Silva has tried to strike a careful balance between pride in her faith and not alienating more secular-minded Brazilians. She said in an interview this month that "the Bible is without a shadow of a doubt a source of inspiration," but immediately added that all of her decisions are "taken on a rational basis."

One campaign jingle celebrates "the faith of every believer and the reason of every atheist."

Last month, her Brazilian Socialist Party issued an official platform supporting gay marriage and making homophobia a crime but Silva quickly reversed the stance after Brazil's best-known televangelist, Silas Malafaia, threatened to withdraw his support for her.

It was a costly embarrassment for Silva, losing her support among young and urban middle-class voters and fueling broader concerns that she flip-flops on major issues.

Rousseff tried to capitalize, announcing after Silva's gaffe that she would push through legislation granting evangelical churches the same tax benefits as the Catholic Church.

Analysts say many evangelical voters are still up for grabs. Brazil's poor depend on social programs introduced by the ruling Workers' Party, and could vote for Rousseff regardless of what their pastor may say.

Rousseff has the backing of Brazil's second-largest evangelical church, the Universal Church of the Kingdom of God. Silva belongs to Assemblies of God, which has more members but is less well-organized.

Rousseff even attended the opening last month of a 10,000-seat, 11-story Solomon's Temple built in Sao Paulo by the Universal Church's leader, Bishop Edir Macedo. A media magnate, Macedo has a fortune estimated by Forbes magazine at $1.24 billion.

Macedo's PRB party backs Rousseff and even has a seat in her cabinet. But he exerts more influence with his ownership of the second largest television network, Rede Record.

Bishop Robson Rodovalho, a physicist who founded a Pentecostal church in 1992 that now has more than 1 million followers and a TV channel, said he expects that Silva still has room to grow her support in coming weeks.

"Much of the Church will converge on her candidacy," he told Reuters before going on stage to preach to the sound of rock music in a darkened church hall lit with discotheque lights.

"Brazil is a real democracy. It's only a matter of time before we have an evangelical president. That's a fact."

(Editing by Brian Winter and Kieran Murray)