By Pairat Temphairojana

KHON KAEN, Thailand (Reuters) - In Thailand's rural heartland, supporters of former prime minister Thaksin Shinawatra say they will focus on winning an election next year, even though they would have to govern on military terms if they win.

Thais handed the junta of Prime Minister Prayuth Chan-ocha a convincing victory in Sunday's referendum on a military-backed constitution, rejected by both major political parties because it would restrict future democracy.

Despite a military ban on campaigning ahead of the referendum and the detention of dozens of activists and students who opposed the charter, the opposition appeared to accept the electoral verdict.

"There will be no protest movement," said Sabina Shah, a former local leader of "red-shirt" Shinawatra supporters in the northeastern city of Khon Kaen.

"We'll wait for the election. It will inevitably be the Puea Thai Party, or whatever they become, winning," she said, referring to the political party that carried Yingluck Shinawatra to power in a landslide in 2011.

The anti-junta opposition has laid low since Prayuth toppled Yingluck's government in a 2014 coup, confident in its ability to win back power at the ballot box and keen to avoid a confrontation with a junta that has quashed any sign of dissent.

The red shirts are supporters of Yingluck and her billionaire brother, Thaksin, who was ousted in a previous coup in 2006.

Thaksin and his allies have won every election since 2001, but have been removed from power either by the courts or in coups during a decade of political turmoil pitting the Shinawatras against the military-royalist establishment.

But even if Shinawatra allies win the 2017 election, they will find it difficult to scrap the new charter or reverse the military's power over future elected governments.

"We are not going to oppose this constitution but we are certain it will lead to trouble," Thanawut Wichaidit, spokesman for the red-shirt United Front of Democracy Against Dictatorship group told Reuters.

"It will be difficult to undo this charter even if a government we want comes to power."

Interviews with officers, including two generals, showed the military's ambition was to make future coups unnecessary by using clauses in the new constitution to weaken political parties and oblige future governments to follow a 20-year national development plan set by the army.

The ruling military council has said the charter will pave the way for a general election in 2017, ensure clean politics and end Thailand's caustic political divide.

Critics say the charter is the military's attempt to make good on its failure to banish Thaksin from Thai politics after the coup in 2006.

"If these laws become permanent, then it will be impossible for the Shinawatras to come back," said Chaloem Butrphu, a farmer who voted against the referendum, speaking in a village among verdant rice paddy fields near Khon Khaen.

"Not a chance."

CONFUSION

Thailand's northeastern Isan is its poorest and most populous region -- and a stronghold of Shinawatra support.

But even there, opposition seems to have weakened. Preliminary results show 51 percent in the region voted against the charter, compared with over 60 percent there who voted "no" in the last charter referendum in 2007.

The desire to see a quick election may have convinced some voters to vote in favor of the charter, despite reservations, said Cherdchai Tontisirin, a former Puea Thai member of parliament for Khon Kaen.

"I do not take this result with a heavy heart," said Cherdchai, who in the past helped mobilize red shirts from Khon Kaen to participate in the street protests that have punctuated the past decade in Bangkok.

"The people of the northeast did not approve the referendum, and some accepted this draft because they wanted to see an election happen soon."

The ban on campaigning and the limited debate on the constitution meant many voters lacked a full understanding of what it contained, said Chuchat Loasuwan, a member of the local election commission outside Khon Kaen.

Chuchat was tasked with explaining the referendum to villagers, but said the law against campaigning made him reluctant to say too much.

"It was uncomfortable," he said. "They wouldn't let us say much because if we did, we might be breaking the law or misleading people."

Thaksin called the constitution a "folly" in the run-up to the referendum and said it would perpetuate the power of the junta and make Thailand ungovernable.

Yingluck said on Monday that she accepted the result but that the charter was undemocratic.

"I'm not surprised at the result of the referendum because there was no chance to show opinions or debate the constitution fully," Yingluck said in a Twitter post on Monday.

"I am sad and disappointed for a country that is going to step backward by using a constitution that looks democratic but in fact is not real democracy."

(Additional reporting by Amy Sawitta Lefevre and Patpicha; Writing by Simon Webb. Editing by Bill Tarrant.)