An ecstatic Marois told supporters in Argenteuil riding north of Montreal that the win by 71-year-old retiree Roland Richer marked "the beginning of the end" for Premier Jean Charest's government, which has been beset by ethical scandals as well as social unrest in the form of student protests.
"Today, the Quebecers of Argenteuil sent a message to Jean Charest," the PQ leader thundered as she pointed out her party had captured a fortress that had been held by the Liberals for almost 50 years. "They said no to corruption, no to Liberal cynicism. They said yes to change, yes to hope.
"We will change the government, we will change direction — and we will change countries," a beaming Marois added to chants by the crowd of "We want a country."
The news wasn't completely bad for the Liberals — they won the byelection in Montreal-area Lafontaine riding, another long-held piece of the party's political real estate.
Marois told the Argenteuil crowd that power belongs to all Quebecers and not just "the Liberals and their friends." Saying she hoped Quebecers voted massively in the next election "to get rid of this corrupt government," Marois said she wants to form an honest government with a clear vision that will enrich every Quebecer and not just a select few.
Despite Marois' bluster, the PQ only nudged up in support from previous elections, with Francois Legault's upstart Coalition For Quebec's Future actually siphoning away some of the Liberals' traditional votes on Monday night.
But Premier Jean Charest wasn't ready to run up a white flag.
He described the result in Argenteuil as a "postponement" of the Liberal victory in the riding.
"We're going to look at the results," Charest said of the Argenteuil outcome as he talked to reporters. "It was a close result. It's not a result that says there was total domination. On the contrary, the election result was very close."
While he said he didn't underestimate any of his rivals, Charest dismissed Legault, a former PQ cabinet minister, as a "sovereigntist," lumping his party in with the PQ.
"Each byelection teaches us something," Charest said. "At the same time, tonight we split the results. The score was very close. We'll analyze the results but the score we had tonight was respectable."
Asked if he felt the ongoing student crisis had an effect on the results, Charest stood by his handling of the tuition fee dispute.
"We have done everything humanly possible," he said, pointing out that Marois had been a strong supporter of the students. Marois told reporters at the PQ's victory party that Charest's handling of the student crisis had been "lamentable."
Charest called the byelections after months of unrest in the province due to student tuition fee protests which have occasionally turned violent.
Nightly demonstrations have been held in several Quebec cities and people have taken to banging pots and pans not only in support of the students but against a law passed by Charest's government that puts tighter limits on demonstrations.
The protest has gotten international attention and draw complaints about Charest's handling of it, although some pundits said the premier's tough approach had won him significant support among voters.
However, Gabriel Nadeau-Dubois, spokesman for the hardline CLASSE student group, tweaked the analysts on Twitter as the results came in.
"Looking forward to hearing from those who were saying the student strike would benefit the Liberals," he tweeted.
As the campaign began, Charest also had the baggage of an inquiry into construction industry corruption and the awarding of public contracts, which Charest resisted calling for almost two years before yielding to public pressure. It recently started its hearings.
Argenteuil had been Liberal since 1966. It had once been the fiefdom of Claude Ryan, the stern intellectual who led the party during the 1980 Quebec referendum won by the federalists.
The Liberals had started with a decent lead in Argenteuil although the PQ crept up during the evening before passing them in an abrupt turnaround.
The PQ had targeted Argenteuil and Leader Pauline Marois had campaigned in the area several times, visiting the last time on Saturday. Charest was there on Friday to back Proulx.
The office of Quebec's chief returning officer put voter turnout in Argenteuil at about 25 per cent. There are a total of 41,000 voters in the riding, 83 per cent of whom are francophones.
The riding had been held by David Whissell, who quit Charest's cabinet to devote more time to his construction business.
Lafontaine riding became vacant earlier this year after the resignation of Tony Tomassi, who quit his seat in the legislature to fight fraud and breach of trust charges.
Tomassi, who was once Charest's family minister, left the Liberal party and caucus in 2010 after allegations of improper use of a credit card issued by a private company to him while he was a member of the legislature.
Marc Tanguay, the Liberal party's president, held the riding although with a reduced majority than the 10,181 votes Tomassi nabbed in the 2008 general election.
Tanguay briefly addressed cheering supporters late in the evening as he accepted his victory.
"Thank you for your confidence," he said in French before expressing his gratitude in English and Italian.