An unflattering ad that stars Parti Québécois Leader Pauline Marois has prompted a legal threat from a videographer, as the spectre of a possible fall election looms over the province.

The political ad, launched by Quebec's ruling Liberal Party, is a highly stylized, black-and-white voiceless video clip that uses amateur footage of Marois at a spring casserole protest in Lachute.

The 15-second video shows Marois banging on two metal lids, in slow motion, looking quizzically at people striking pots and pans around her. At one point, she fumbles with her lids.

Guy Séguin, the man who originally shot the video and posted it on Facebook, has sent a legal letter to the Liberal Party asking them to stop using the clip.

He recorded the images in Lachute during a byelection campaign that saw the PQ score a surprise victory in the longtime Liberal stronghold riding of Argenteuil.

The footage was originally posted to Facebook, on the PQ's official website and YouTube.

PQ members accused the Liberals of using negative "Republican-style" tactics by running the ad.

Premier Jean Charest defended the clip, stressing that the PQ "chose themselves to make these pictures public" in the first place.

"This illustrates an episode in the political life of Pauline Marois that Quebecers have an interest in knowing," Charest told reporters Tuesday.

"The image speaks for itself. We did not suggest a conclusion Quebecers should reach -- given how obvious that conclusion is."

In the past, Quebec Premier Charest has mentioned he associates Marois with "the streets and protest," constantly reminding people of her support of the student strike.

After vigorously encouraging the student movement this past spring, Marois has backed away, at least symbolically.

She wore the movement's iconic red felt square for months, until last week, when she stopped pinning it on her lapel.

A Quebec election could be held as early as September, if Charest chooses to call one this summer. The premier has until the fall of 2013 to call a vote.

The Liberals' possible election campaign could prominently feature the PQ and its support for Quebec's student strike.

Charest released his own video last week, in which he directly addresses Quebecers and underlines his government's "political courage" during "this period of turbulence."

With files from the Canadian Press