The battle for the "Bible belt", one of the most crucial constituencies in the Republican White House race, will begin in earnest in Waterloo, Iowaon Sunday, where Texas governor Rick Perry, who announced his candidature on Saturday, is to speak at a dinner in the Electric Park ballroom that will also be attended by congresswoman Michele Bachmann.

Bachmann changed her diary to be there, setting up an early showdown between two Christian evangelicals either of whom could be Barack Obama's opponent in the presidential election next year. Bachmann received a big early advantage on Saturday night when she came top of the Republican straw poll in Ames, the biggest political festival in America this year. In the second biggest vote in the history of the straw poll, with almost 17,000 voting, Bachmann took 4,823. Perry, as he had not declared in time, was not on the ballot but still managed to win 718 write-ins. The victory provides Bachmann with a short-term advantage but it is unlikely to last long with the arrival of Perry. The real race begins now.

The importance of the evangelical vote is huge, representing an estimated 40% of Republicans who will vote in the Iowa caucus, which is scheduled for February. Iowa, as the first of the contests, matters – helping to propel candidates to the front of the race and seeing others heading for oblivion.

Bachmann has received the endorsement of more than 100 pastors and Christian leaders in the state in the past week alone. But Perry's entry upsets her calculations. He is both a politician and part-time preacher, the kind of southerner who appeals to the Christian right. "Perry's entry shifts the dynamic," said Steve King, a rightwing congressman from Iowa, who was speaking at the Iowa state fair, where a string of Republican candidates used soapboxes to address voters sitting on straw bales.

King is close to Bachmann but said he would not endorse anyone until after Labour Day, on 5 September.

Other Republicans in the race such as Rick Santorum, Tim Pawlenty and Herman Cain have made bids for the Christian right, but it is Bachmann who appears to have won them over with her anti-gay rhetoric and strong views on abortion and other social issues.

Perry, however, may be more than a match for her. He arrives fresh from a prayer rally last weekend at a stadium in Houston, Texas, that attracted 30,000 participants and was backed by evangelical leaders in Texas, Mississippi and elsewhere in the south. The influence of those southern leaders, who head megachurches, run networks of radio stations and have access to millions of dollars in funds, extends into Iowa and other midwest Bible belt states.

One danger for the Republican right is that Bachmann and Perry could split the evangelical vote, allowing Mitt Romney, the former governor of Massachusetts, to win Iowa. Romney, though right of centre, is detested by the Tea Party movement and seen as a flip-flopper. He is regarded as inconsistent on social issues, and, for some on the Christian right, another negative is that he is a Mormon. King agreed that a Perry-Bachman fight could split the vote and let Romney through. "That is possible. There are many scenarios. That is the likely one," he said.

The return to a pivotal place in US politics for the Christian evangelicals comes after a short gap in which social issues, so dominant in the Bush era, with battles over abortion, gay rights and stem cell research, were pushed to the side by concerns over the economy. Gay rights and abortion were not issues in the 2008 election between Obama and his Republican rival, John McCain.

Just how raw the issue of gay rights is was evident when Bachmann spoke at the Iowa state fair. Hundreds gathered to listen. She spoke for only three minutes, then suddenly stopped and rushed away, heckled by Gabe Aderhold, a gay activist. "You treat me like a second-class citizen," he shouted. "Shame on you."

Aderhold, 17, turned on Bachmann's husband, Marcus, who runs a clinic in Minnesota where gay people are allegedly encouraged to pray to become heterosexual. "You can't pray the gay away," Aderhold shouted at him.

The Iowa supreme court ruled in 2009 that a state ban on same-sex marriage was unconstitutional. Rightwingers, including King, and Christian evangelicals, backed by huge spending on radio and television ads, mounted a successful campaign to have three of the judges who voted against the ban removed.

Robert Haus, a veteran Iowa campaigner who was involved in the 2008 election, argued that the power of the evangelical vote was exaggerated. "The Christian evangelical bloc is a term that is overused and is not accurate. They are sophisticated, do their research on candidates and make up their own minds," Haus said.

Andy Pierce, 67, a Bachmann supporter, disagreed: "The Christian evangelicals are so important," he claimed. Pierce said he watched in amazement in his home town of Ottumwa, Iowa, as pastors and church members came out in force to back Mike Huckabee, the former governor of Arkansas and a Baptist preacher. "They came out against Romney because he is a Mormon," Pierce said, enjoying a drink at a beer tent.

Huckabee, campaigning on a shoestring budget, won the Iowa caucus, even though Romney spent tens of millions of his personal fortune campaigning there. A bass player, Huckabee was scheduled to perform at the straw poll in Ames, a traditional test of candidates' popularity and organisational skills, with supporters bussed in from around the state. Huckabee, though he will be joined on stage by Cain, has not yet said who he will endorse. Romney, knowing he cannot win, opted not to take part.

Pierce's wife, Joanne, likes Romney but is pessimistic about his chances. "I do not think Romney will get in because he is a Mormon. It does not bother me but the Christian right will not support him," she said. "I don't know much about Rick Perry but I have a feeling he is going to be the one."

There is still an outside chance that the battle for the Bible belt could become a three-way race. Sarah Palin, who was also at the state fair on Friday, attracting the biggest crowds of all the politicians, was mobbed as she moved from stall to stall, telling reporters she still had not made up her mind whether to stand.

Romney will be the happiest politician watching this weekend's events unfold. But that may change if the early excitement about Perry persists and he knocks Bachmann aside. The determining factor, apart from religion, is who has the best chance to oust Obama. One of the T-shirts at the state fair had "Hope and Change", Obama's 2008 slogan, on the front, and "I hope we change our president" on the back.