Simon-Pierre Diamond, left, stands with Premier Jean Charest on Monday after announcing he will be running for the Liberals in the Vachon byelection. (CBC)

The Quebec Liberal Party is pinning its hopes in an upcoming byelection on a former Action Démocratique MNA, Premier Jean Charest announced on Monday.

At the age of 22, Simon-Pierre Diamond became the youngest person ever elected to the national assembly when he won the Marguerite-D'Youville riding in the 2007 election.

Now 25, Diamond says he will run for the Liberals in the riding of Vachon, south of Montreal.

ADQ, Liberals facing tough times

The announcement comes at a time when the ADQ is struggling. With only four MNAs remaining in the national assembly, the party has just emerged from an acrimonious quarrel over leadership.

However, times are also tough for the Liberals, who are facing attacks from opposition parties over ethics concerns.

The government is facing repeated demands for a public inquiry into allegations of collusion and corruption in the construction industry, as well as political party financing practices.

On May 5, Charest was forced to fire his family minister, Tony Tomassi, after he admitted to using a credit card belonging to a private security firm for his personal use.

And in April, Charest launched an inquiry into the process the province uses to select judges after Marc Bellemare, his former justice minister, claimed he had been pressured to name Liberal-friendly candidates to the bench.

Diamond said he had received a personal invitation from the premier to run for the Liberals. He said he accepted because he has grown uncomfortable with the ADQ's social conservatism and what he calls fear-mongering on public security issues.

Harsh reaction

Some critics have called Diamond's decision to switch parties ironic. When two MNAs crossed the floor to join the Liberals shortly before the last election, Diamond described them as political prostitutes.

"I admit that those are words that were harsh," Diamond told reporters on Monday.

Diamond's defection does not sit well with his former ADQ colleagues.

"I told him, 'You will lose!'" MNA Sylvie Roy said.

PQ MNA Stéphane Bédard said the party's decision to choose Diamond as its candidate is a sign the Liberals are in hot water.

"I'm not surprised to learn that Mr. Charest isn't capable of finding Liberals who want to run for him," Bédard said.

However, Charest said he is confident Diamond can win thanks to what he said is the Liberal Party's focus on the economy.

"The riding of Vachon has a lot of young families, and the No.1 issue confronting Quebec today is the economy," the premier said.

The riding of Vachon was left vacant by former Parti Québécois MNA Camil Bouchard, who resigned in December. The seat has been held by the PQ since 1994.

Diamond lost his seat to Monique Richard of the PQ in the last provincial election in December 2008.

The premier has yet to set a date for the byelection.

Environmental activist Martine Ouellet is running for the PQ, while former Saint-Lambert councillor Alain Dépatie is running under the ADQ banner. Union representative Sébastien Robert will represent Québec Solidaire.