Harper actively solicited names for the Conservative party's mailing list via his @pmharper Twitter account, viewed by many Ottawa observers as the official social media voice for the Prime Minister's Office.
And since government policy forbids the use of official communications for partisan purposes, Monday's tweet would appear out of line — as would several other previous tweets in the same vein in recent months.
But the account doesn't fall under government communications rules, Harper spokeswoman Julie Vaux said in an email.
"@pmharper is a political Twitter account, not a government one," Vaux said, noting Harper runs a second, government account at @pmwebupdates.
That sent eyebrows shooting up all over Parliament Hill.
The @pmharper account is defined as belonging to the Prime Minister of Canada, with no reference to the party. It also includes a link to the prime minister's official government website.
It has has amassed more than 340,000 followers since it was set up in July 2007; updates are posted from it several times a day.
On Monday, after tweeting the link to sign up for party mailings, Harper wished a fellow Conservative MP a happy birthday, noted his government's support for an anti-bullying project and sent out a photo of himself meeting with the new governor for the Bank of Canada.
By 6 p.m., nothing had appeared on @pmwebupdates, which is described on the site as being the "latest web updates from Prime Minister Harper's website" and has slightly more than 8,000 followers.
In the month of May, there were nine tweets, including four photos of cats up for adoption, a podcast of a recent speech, video of an announcement on beef exports and photos of Harper with the president of Chile.
"Once again, the Conservatives are guilty of a dangerous mix," said NDP MP Alexandre Boulerice.
"There is less and less a distinction between the work of government and the political party, which does partisan work."
The Conservative government has endured fierce criticism in the past for applying a partisan stamp to what are supposed to be non-partisan communications — using the phrase "Harper government" instead of "Government of Canada," for instance.
Where social media is concerned, other world leaders make a much clearer distinction between their political and government identities.
In the case of U.S. President Barack Obama, the @BarackObama account is identified as being run by political staff and links to a political, not presidential, website. The account also notes that "tweets from the President are signed '-bo.'"
Obama's office has a separate White House account, described as "the latest news and ways to engage with President Obama and the administration."
U.K. Prime Minister David Cameron's account is @David_Cameron and is described as "official profile of David Cameron, Leader of the Conservative Party and Prime Minister of the United Kingdom."
It notes the account is run by him and his party, while official tweets from his government come from the @Number10gov, described as "the official twitter channel for Prime Minister David Cameron's office, based at 10 Downing Street."
It's not clear who is doing the tweeting from Harper's account; his office has said he posts himself only "occasionally."
Vaux did not respond to questions Monday on whether Harper's political account is handled by a government staffer paid by taxpayers or by the Conservative party.
The @pmharper account received a digital makeover in the last year, moving from posting only links to press releases to making an effort to provide a more personal touch.
Whomever is behind the account has made past references to the TV show Seinfeld, and even bantered with the Homer Simpson account last year when the Mayan-predicted end of the world failed to materialize.
In January, the account was used to chronicle a day in Harper's life as prime minister.
At the time, Vaux said the decision was made to provide a glimpse behind the scenes because they often received the question of what such a day looked like.
"So we thought we'd use social media to show people what it's like as we start this parliamentary session,'' Vaux said at the time.
None of the "day in the life" tweets appeared on the official government account.
Liberal MP Bob Rae said what Harper posts online needs to be understood as being from both the prime minister and head of a political party.
"You can't say that the prime minister of Canada isn't also the leader of the Conservative party," Rae said.
"It's the same person."
But he cautioned that Harper's not the only one guilty of appearing to mix the two online, suggesting everyone appears to be mixing some partisanship with their public roles.
"I don't see a big deal there," he said.
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