OTTAWA - The Conservatives' claim to clean and transparent governance suffered an embarrassing blow Tuesday when the RCMP raided the governing party's national headquarters at the request of Canada's elections commissioner.

Prime Minister Stephen Harper confirmed the raid was connected to a protracted legal battle between the Conservative party and Canada's elections watchdog over alleged spending irregularities during the 2006 election campaign, but he insisted his party had done nothing wrong.

Elections commissioner William Corbett asked the Mounties to execute a search warrant, but officials wouldn't say why.

"I can confirm that the commissioner of Elections Canada has requested the assistance of the RCMP in the execution of a search warrant," said spokesman John Enright.

"The commissioner has no further comment."

The raid could pose a significant political threat to the Conservatives. Harper owes his mandate to voter disgust with the Liberal sponsorship scandal and the announcement of an RCMP investigation into a leak at the Finance Department before changes were made to income trusts by the Liberals.

Voters who turned to the Conservatives two years ago after Harper promised to sweep away Liberal corruption were bombarded Tuesday with images of police officers searching for evidence of wrongdoing at Conservative party offices.

The prime minister was adamant that his party has done nothing wrong. He described the affair as a difference of interpretation over election spending laws and expressed confidence that the party's legal position is "rock solid."

He also suggested that the raid occurred just before Conservative party lawyers were scheduled to question Elections Canada officials for a separate civil suit the Tories have launched. He did not explain why the timing was significant.

"While today's actions may or may not delay that somewhat, we remain extremely confident in our legal position," Harper said.

The party itself issued an understated news release saying that "Elections Canada visited" Conservative headquarters, with no mention of the RCMP.

At least two Mounties sifted through party offices on the 12th floor of a downtown office building as camera crews captured the images outside. A short time later, two officers rolled a cart full of boxes and bags into a 17th-floor mailroom. Elections Canada official Andre Thouin was seen leaving the building with a carton of documents.

The Liberal party had their own camera trained on the raid - no doubt to gather footage that will be used in campaign advertising. Liberals - who were hounded out of office by the sponsorship scandal and the income trust investigation - were revelling in the chance to turn the tables on Harper.

"This (police raid) is what we get when we play fast and loose with election law," deputy Liberal leader Michael Ignatieff told the Commons.

"This is what we get when we cheat and we get caught."

Corbett launched an investigation in April 2007 into $1.2-million worth of Conservative television and radio advertising that was billed to individual Tory candidates even though the ads were virtually identical to national party advertising.

In a separate but related civil action, the Conservative party took Elections Canada to court to challenge Mayrand's refusal to reimburse Conservative candidates for part of the advertising money when they claimed it as local expenses. The ads were produced for the party's national campaign, which has a separate limit for election spending.

Peter Van Loan, the government's House leader, questioned why the elections commissioner felt compelled to call in the RCMP. He said the Tory party has "always been forthcoming" with any document the watchdog has requested.

A Conservative party official, speaking on condition of anonymity, later called the raid "a PR stunt" and "an intimidation tactic" aimed at deflecting attention from Elections Canada's weak case against the party.

However, Liberal MP Dominic LeBlanc said Harper is confusing the civil case with Corbett's separate "quasi-criminal" investigation into possible wrongdoing. He doubted Corbett would have called in the RCMP unless the Conservative party was refusing to co-operate or he had reason to fear the party was about to begin shredding evidence.

LeBlanc further charged that Harper must be directly implicated in the spending irregularities during the last campaign.

"It's inconceivable that a scheme like this was dreamed up and the prime minister wasn't intimately aware," LeBlanc said, noting that Harper is an expert in election spending laws, having led court challenges against restrictions on third party advertising in the past.

Liberal Leader Stephane Dion brushed aside suggestions that the raid might give his election-wary party the ammunition it's been waiting for to finally topple Harper's minority government.

"I'm not in the mood to speculate about elections today. The prime minister needs to answer very, very serious allegations," he intoned sombrely.

New Democrats and the Bloc Quebecois piled on, mocking Harper's promise to run a squeaky-clean government.

"Conservatives campaigned that they were going to be as pure as the driven snow," said NDP Leader Jack Layton.

"Well, it turns out that it is not much different over here (on the Tory side of the Commons) than it was over there (on the Liberal side) and the RCMP has had to be called in again."

According to documents filed in court by Elections Canada, some Tory campaign officials told the elections watchdog that the scheme was referred to as the "in-and-out" plan.

Under the plan, party headquarters would send as much as $50,000 in cash to candidates across the country. The candidates would then give the money back to headquarters, claiming they were paying for advertising.

In some cases, the advertising was virtually identical to national ads, the only difference being a barely discernable tag line that listed the names of local candidates.

The RCMP, which has faced accusations of interfering in the last campaign by announcing that the force was launching an investigation into the income trust matter, took pains Tuesday to point out that it was simply acting at the behest of the elections commissioner.

"It is not an RCMP investigation. We're there to assist, but that's it," said RCMP Cpl. Jean Hainey.

He would not provide any other details.

Harper has knocked heads repeatedly with Elections Canada. As head of the National Citizens' Coalition he spearheaded a court challenge to election spending restrictions on lobby groups.

Two years ago, the former chief electoral officer, Jean-Pierre Kingsley publicly contradicted the prime minister on the matter of whether political convention fees must be reported as donations. The Conservative party was later forced to admit it had not accounted for the fees properly.

And last summer, Harper took Mayrand personally to task for refusing to require veiled women to show their faces to vote in federal byelections. In fact, the law does not specifically require visual identification of voters, something the Tories are now proposing to rectify.