MONTREAL - All four parties in the Quebec National Assembly condemned the Harper government today, following Ottawa’s decision to challenge the province's referendum law.

The Parti Quebecois launched the opening salvo from the steps of the assembly.

Alexandre Cloutier, the PQ's minister of intergovernmental affairs, called the move a "direct" and "devious" attack against the Quebec nation.

Cloutier said only Quebec and its own legislature, the National Assembly, have the right to determine whether the province stays in Canada -- not the federal government.

"Obviously, they want to make sure we'll never get our country," Cloutier said Sunday at a news conference in Quebec City.

Bill 99 declares that a referendum vote that gets 50 percent plus one in favour of independence should be considered enough to separate from Canada.

The federal government finally filed a legal intervention against the law last week, entering into a court challenge launched by Equality Party leader Keith Henderson and lawyer Brent Tyler shortly after it was first adopted.

The lawyer said that he is delighted that the Harper government is now helping out with the cause.

“We’re very happy, obviously having a private citizen with his meager resources going up against the Attorney General of Quebec, it's always desirable to have an ally,” said Tyler.

Tyler explained his objection to the Quebec law.

"One of the problems with Bill 99 is it gives leave of effect to a purely consultative move and empowers the province of Quebec to proceed with independence without following the amendment formula under the Canadian constitution," said Tyler.

For years, the federal government's strategy appears to have been to delay the court case. But with a trial date to be set this December in Quebec Superior Court, Ottawa decided to enter the fray.

The federal government argues in its intervention that Bill 99 does not make provide the legal basis for "unilateral declaration of independence by the government."

One federal government representative downplayed the intervention Sunday.

"We don't want to change anything. We don't want to change the constitution at all," said federal MP Denis Lebel.

The dispute could revive constitutional questions that have remained mostly dormant for more than a decade.

While support for independence is low in Quebec, the PQ could use a battle with Ottawa to drum up support in an election, which could be called as early as next month.

Bill 99 was introduced in the year 2000 by Lucien Bouchard's PQ government in response to the federal Clarity Act, which states that support from a clear majority is required in a referendum vote to begin the process of secession.

While the PQ bill did not, in the final vote, receive the support of the Charest Liberals they expressed agreement with some of its fundamental principles. Mulcair was part of Charest's caucus.

Describing the law "one of the most important in Quebec history," Cloutier said he will instruct government lawyers to do everything they can to fight the court challenge.

He added the PQ will introduce a motion next week in support of Bill 99 and expects the support of all political parties.

But while Cloutier came out forcefully on Sunday, his government's own lawyers have taken a far different approach in addressing the court challenge.

In fact, a defence filed in the case recently by the Quebec government argues Bill 99 doesn't conflict with the Canadian constitution.

Instead, it contends that Bill 99 reaffirms the "fundamental rights" of Quebecers that are consistent with Canadian law.

A spokesman for Harper, Carl Vallee, said Saturday he anticipates the PQ will use the issue to revive old quarrels.

He said the prime minister's position on the issue is well known.

Meanwhile, Tom Mulcair, the head of the opposition New Democrats, said he remains committed to his party's official policy: a Quebec referendum vote of 50 per cent plus one is sufficient for secession.

The bulk of the NDP's seats are in Quebec.

Speaking with reporters Sunday in Trois-Rivieres, Que., Mulcair suggested the party policy avoids the kind of wrangling he expects to see from the PQ and the Harper government.

"There's no discussion on that issue," he said.

With a file from The Canadian Press