OTTAWA - The brain trust at NDP headquarters had a tricky problem to solve last week.

As lousy economic news piled up, the NDP wanted to amp up attacks on Prime Minister Stephen Harper.

But they had to do it in a way that would avoid making those who put him in office feel stupid for having voted for him.

“It’s like a Rogers customer telling a Telus customer who they’re trying to get to switch saying you’ve been an idiot for five years by being with Telus, now come over to us,” said pollster David Coletto, the CEO of Abacus Data.

If the NDP can win over this increasingly significant group of voters — the Blue-Orange Switchers — they’ll sit on the government benches for the first time in history.

Conversely, Harper must hold the support of that group to stay in 24 Sussex.

In his firm’s most recent survey, from earlier this month, Coletto found 9% would consider voting NDP or Conservative but will definitely not vote Liberal.

“There’s far more Conservatives open to voting NDP,” Coletto said.

That’s why New Democrats must be careful about the language they use to criticize Harper. This voter, after all, still likes Harper.

“The NDP learned that from how they dealt with the Bloc Quebecois in Quebec. They made the conscious choice in 2011 not to attack outright the Bloc. They didn’t want to tell Quebecers that for all these years, you’ve done the stupid thing by voting for a party that could never win. They simply said you could do better.”

One thing these voters do agree on: They don’t like Liberal leader Justin Trudeau.

Abacus found that just 4% had a positive evaluation of Trudeau while 72% had a negative evaluation.

By comparison, when these Blue-Orange Switchers were asked to evaluate the other leaders, 52% gave Harper a thumbs up and 43% did the same for Mulcair.

And just 4% gave Mulcair a negative rating while Harper’s negative number was 14%.

But who will these Blue-Orange switchers vote for?

Abacus found 39% of Blue-Orange switchers are ready to vote Conservative, 26% would go NDP but a big chunk — 28% -- are undecided. They just know they won’t vote Liberal.

Tories and New Democrats will fight for undecided Blue-Orange Switchers in ridings in Quebec City, in factory towns in southwestern Ontario, in prairie cities like Winnipeg, Regina, Saskatoon and Edmonton and all over the increasingly important battleground province of British Columbia.

A Vancouver-based pollster who is an expert on BC voters says the biggest pool of potential blue-orange switchers is in the suburbs around Vancouver — Port Moody, Richmond, and Surrey.

“If these voters feel vulnerable or the election is driven by left of centre issues — health, environment — they will consider the NDP. If they feel the Conservatives have lost touch, there are enough blue collar voters and populists to make a difference,” the pollster said on condition he not be identified.

There’s no doubt the NDP rise in the polls has been powered by pitches to what pollster Frank Graves calls the “promiscuous progressive.”

These are those Liberal-NDP switchers who like $15-a-day childcare and a hike (small as it may be) in corporate taxes.

But to steal seats from Conservatives, Mulcair will focus on populist parts of his platform, the parts that have that sticking-up-for-the-little-guy appeal.

He’ll cut taxes for small business. He’ll bring big banks, cell phone providers, cable companies and airlines to heel with more consumer-friendly policies.

New Democrats have been polishing this populist appeal for more than a decade.

I once watched Mulcair’s predecessor, the late Jack Layton, stand up in a packed hall in Nanaimo, B.C. in the 2006 election campaign and warn voters there that Harper had lost the populist touch, that while the Reform Party cared for the grassroots, Harper’s Conservatives now thought more about corporate fat cats than families struggling to make ends meet.

A week later, I watched Harper campaign on Vancouver Island and his message to these same voters was a mirror image of Layton’s.

The NDP used to be a party of the people, Harper said, but they had lost their populist touch and now represented downtown elites and thought more about union bosses than they did working men and women.

As this summer campaign continues, you’re certain to hear this rhetoric repeated as Mulcair and Harper duel for this vital populist voter.