Attorney Christopher McGrath launched his State Senate campaign Wednesday, telling several hundred supporters in Inwood that the April 19 special election to replace convicted former Majority Leader Dean Skelos is “the most important election in the history of New York State.”

McGrath, a Republican from Hewlett, said the race against Assemb. Todd Kaminsky (D-Long Beach) could determine the balance of power in the Senate, now controlled by Republicans and a coalition of independent Democrats.

McGrath said when Democrats controlled the Senate in 2009 and 2010 — along with the Assembly and the governor’s office — Long Islanders saw a new MTA payroll tax and the elimination of property tax rebate checks.

“If you believe in a two-party system of government like I do, then I need to win this election,” said McGrath, who pledged to fight for increased school aid and against Common Core standards.

Hempstead Town Supervisor Anthony Santino called the race a “crusade” to protect Long Island. A win for Kaminsky, he said, would turn “the entire mechanism of state government over to the [New York City mayor] Bill de Blasio far-left wing New York City Democrats.”

Born and raised in Inwood, McGrath is a personal injury attorney and partner with the law firm of Sullivan, Papain, Block, McGrath & Cannavo based in Garden City and a former president of the Nassau County Bar Association.

McGrath, who declined to answer questions at the rally, said that if elected he would keep his legal job. Gov. Andrew M. Cuomo wants to cap outside incomes for lawmakers — who are considered part-time — at 15 percent of their base annual salary of $79,500.

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Mike Murphy, spokesman for Senate Democrats, called McGrath the “ultimate agent of the corrupt status quo” for saying he would continue to work as a lawyer if elected. “While Long Islanders insist on cleaner government, Chris McGrath proves he is just another tool of Dean Skelos’ corrupt political machine,” Murphy said.

Skelos, of Rockville Centre, was convicted in December of eight counts of bribery, extortion and conspiracy and plans to appeal.

McGrath said he liked Kaminsky, a first-term assemblyman and a former federal prosecutor, “and I am going to like him more as the assemblyman working alongside me in the Senate.”

Green Party candidate Laurence Hirsh, an accountant from Valley Stream, also entered the race this week.