By Tamim Elyan

CAIRO (Reuters) - A legal case against the assembly writing Egypt's new constitution was referred to the country's top court on Tuesday, a move likely to give the Islamist-dominated body enough time to complete its work before the judges can rule.

The step appeared to remove legal doubts overshadowing a process that will shape the post-Hosni Mubarak era. But the assembly still faces a struggle to build consensus around a text that is exposing fault lines in Egypt's new political landscape.

"The case is finished. The challenge will now be a political one, not a legal one. If you don't have a consensus you will have a big crisis," said Hassan Nafaa, a professor of political science at the University of Cairo.

The new constitution is a major component of a transition from military-backed autocracy to a democratic system of government that Egyptians hoped would follow the popular uprising that swept Mubarak from power last year.

Yet its drafting has been marred by political bickering. Non-Islamists say the process has marginalized other voices in society. Much of the debate has been shaped by a tussle over the role Islam should play in government.

There are also complaints the text falls short of hopes for revolutionary change sought by parties that were due to launch a new initiative to press their demands - a sign of a push to shape the debate.

The judge hearing 43 complaints against the way the assembly was formed sent the case to the Supreme Constitutional Court. Plaintiffs, many of them motivated by alarm at the Islamists' sway, argued the 100-person assembly had been formed illegally.

Legal experts said it could take months - up to six by some estimates - for the constitutional court to examine the case. Barring an exceptional burst of activity by the judges, that means the assembly will have time to finish the constitution by December's deadline. The text will then put to a referendum.

"The decision gives the assembly the chance to finish what it started by completing the draft and putting it to a referendum," said Abdel Moneim Abdel Maqsoud, a Muslim Brotherhood lawyer, at the end of a chaotic court session punctuated by chants for and against the assembly.

"Once the constitution is approved in the referendum ... the Supreme Constitutional Court has no authority over it."

"CRUNCH TIME"

The constitution has been the focus of political and legal struggle since the start of the year. The Muslim Brotherhood and the Nour Party - both Islamist groups - secured an influential say over the process by winning a majority in the first parliamentary election held after Mubarak's removal from power.

But the assembly formed shortly afterwards was dissolved by a court ruling in March after plaintiffs fought a successful legal battle over its make-up. Subsequently, parliament itself was dissolved. New legislative elections are scheduled to take place after the constitution is passed.

According to an October 14 draft, the new constitution will guard against the one-man rule of the Mubarak era and institutionalize a degree of civilian oversight - not enough say the critics - over the military establishment that had been at the heart of power since a 1952 coup.

The draft has been criticized for failing to provide enough protection for rights such as the freedom to form trade unions, which it links to unspecified future legislation. Experts say the vagueness of some of the articles smacks more of the autocratic past than a democratic future.

Highlighting the concerns of the non-Islamists, the liberal Dustour Party headed by Mohamed ElBaradei and the Popular Current, a leftist party founded by defeated presidential challenger Hamdeen Sabahi, are launching a campaign for a "revolutionary constitution", a Current spokeswoman said.

Zaid Al-Ali, a constitutional lawyer who has been monitoring the process, said it was now or never for Egyptians to reach a consensus on the new constitution. "It's crunch time," he said.

"They have two months to rebuild trust between each other, iron out the inconsistencies and gaps in the draft text, and convince the country that the coming constitution is the best available solution for the country," he told Reuters.

Ahmed Said, head of the liberal Free Egyptians' Party, said President Mohamed Mursi must now intervene to appoint a more balanced assembly, indicating that street protests were the only path left for people seeking to change the assembly.

"The issue went out of the arena of the courts and has become one of the people," he told Reuters.

(Additional reporting by Tom Perry and Yasmine Saleh; Writing by Tom Perry; Editing by Alison Williams)