The Senate expenses scandal may be ensnarling Stephen Harper in controversy, but polling suggests it's so far had little impact on Conservative support among decided voters.

Still, very minor changes in support over the last week have tightened the three-way horse race between the major political parties: all are now within one percentage point of each other.

An Ipsos Reid survey released on the eve of the Tories' latest policy convention in Calgary shows Mr. Harper's Conservatives at 30 per cent in the polls, down one percentage point from a week earlier and two points since September.

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"I don't know that you could interpret what is going on with the number as any impact," Ipsos Reid pollster Darrell Bricker said. "Basically what you're seeing is fluctuation within the margin of error."

The Liberals under Justin Trudeau are down two points since last week and stand at 31 per cent.

The major beneficiary of the scandal seems to be the NDP so far. Tom Mulcair's party, the Official Opposition in the Commons, has gained four percentage points since last week to sit at 31 per cent in the polls.

The results are from an online poll of Canadian respondents Oct. 25 to 28.

Mr. Bricker said the scandal appears to be having little effect these days on the government. "It's a wonderful story for the Ottawa press gallery … but the truth is the impact on the Canadian electorate, essentially what's happening is it's reinforcing impressions that people already have of Harper."

"It hasn't raised or elevated either [opposition] party, the Liberals or the NDP, to such a place of prominence where they can actually be in a position to defeat the government and form a majority."

The survey period however doesn't capture reaction beyond Monday, when beleaguered Senator Mike Duffy aired more Conservative Party dirty laundry on the scandal.

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Mr. Harper's government found itself knocked back on its heels in Parliament this week as it scrambles to impose stiff sanctions on three ex-Tory senators accused of padding their expense claims.

The Conservative convention begins late Thursday and Mr. Harper will deliver a keynote address to Tories on Friday.

Mr. Duffy alleged Monday that the Prime Minister's Office coached him to lie about where he got $90,000 to repay questionable expense claims, insisting they told him to explain it as a loan from the bank rather than revealing that it came from the personal bank account of Nigel Wright, at the time the top aide in the PMO.

Mr. Bricker said he hasn't seen much movement in federal Conservative Party support in recent months.

"This is something we've been tracking for some time and we haven't seen an appreciable movement in the government's numbers for quite a while. If you look outside of election periods, Stephen Harper typically scores between 30 and 35 [per cent]," the pollster said.

He said the NDP's persistent strength in the polls makes the party a factor to watch. "The only thing we've seen that I think is of particular interest in all of this is the NDP strength hanging in there. The fact the NDP has got a new floor that is at least 25 [per cent] makes them definitely a spoiler in whatever goes on in the future."

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Mr. Bricker notes that Mr. Harper's personal approval rating is 40 per cent – a number that's slipped just one percentage point since July.

"At 40 per cent – that's enough," the pollster says. "Typically governments that have approval levels of 40 [per cent] – and they're incumbent governments – typically win" re-election.

Mr. Bricker hastens to add that none of this suggests the Harper government is faring well these days.

"I wouldn't want to be misinterpreted as saying these are good days for him – they are definitely not."

He notes the scandal has made it very hard for the government to generate media coverage of recent achievements such as the Canada-European Union trade deal clinched in recent weeks.

The Oct. 25-28 poll has a credibility interval of plus or minus 3.4 percentage points.

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The percentage of respondents who believe Mr. Harper's government deserves re-election is 30 per cent – unchanged from July.

In Ontario, the Conservatives are at 33 per cent, the NDP at 33 per cent and the Liberals at 32 per cent – the tightest race across the country.

In Quebec, the NDP stands at 32 per cent, the Liberals at 30 per cent, the Bloc at 25 per cent and the Conservatives at 12 per cent.

In British Columbia, the NDP hold an edge over the Conservatives – 36 per cent to 29 per cent – while the Liberals are at 28 per cent.

The credibility interval on the regional and province sub-samples ranges from plus or minus 3.8 percentage points to plus or minus 9.4 percentage points.