A poll released Saturday shows Republican Christopher McGrath with a lead over Democratic Assemb. Todd Kaminsky in advance of Tuesday’s special election to replace former state Sen. Dean Skelos.

The Siena College poll of 796 likely voters in the 9th Senate District shows McGrath, an attorney from Hewlett, leading Kaminsky (D-Long Beach) 51 percent to 43 percent, with 5 percent of voters undecided. The poll, conducted between Tuesday and Thursday, has a margin of error of 3.6 percentage points.

A Siena poll conducted in mid-March showed Kaminsky, a former federal prosecutor, with a 47 percent to 45 percent advantage with 8 percent of voters undecided.

Siena College pollster Steven Greenberg said McGrath has made inroads during the past month with registered Democrats and independents and among voters in Kaminsky’s 20th Assembly District.

“We have seen significant movement for Chris McGrath, but this is still a special election — a funky special election occurring on the day of New York’s presidential primary that could have low turnout,” Greenberg said.

In March, 82 percent of registered Democrats supported Kaminsky while 10 percent backed McGrath. Now, 77 percent of registered Democrats in the Senate district back Kaminsky and 19 percent support McGrath.

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McGrath, meanwhile, has gained 8 points in the past month among registered Republicans and 7 points among voters unaffiliated with any major party, the poll shows.

McGrath has also cut into Kaminsky’s once sizable lead in his 20th Assembly District, which is heavily Democratic and covers about 40 percent of the Senate district.

In March, Kaminsky had an 18-point lead in the 20th, which stretches from Long Beach to the Five Towns. The new poll shows McGrath with a 1-point advantage in the Assembly district.

“The Siena poll shows that Chris McGrath has all the momentum, all the energy and is poised to win this important special election for hardworking Nassau County taxpayers and their families,” said Senate Republican spokesman Scott Reif. “Voters are smart, and they understand what’s at stake on Tuesday.”

But Kaminsky’s campaign questioned Siena’s poll and provided Newsday with internal polling that shows the Democrat up 46 percent to 43 percent over McGrath will 11 percent of voters undecided.

“Every poll in this race, including our own internal polling, has shown Todd Kaminsky with a consistent lead because voters are fed up with the corrupt Skelos machine putting themselves before taxpayers,” Kaminsky campaign spokesman Evan Thies said. “Todd Kaminsky is the only candidate in this race with a real plan to fight corruption and has always stood up for Long Island.”

Thies also questioned a 22-point drop in the popularity of Gov. Andrew M. Cuomo from Siena’s poll in March to its April poll.

Kaminsky’s favorability ratings have also taken a hit during the campaign, Greenberg said. The campaigns have sponsored negative ads and mailers. A political action committee formed by StudentsFirst, a nonprofit that supports charter schools, spent $1.2 million last week on TV ads and polling against Kaminsky.

Four weeks ago, Kaminsky had a 44 percent favorable and 20 percent unfavorable rating in the Senate district. Now, 41 percent view him positively, and 49 percent have a negative opinion, the poll shows.

McGrath has a 48-39 percent favorability rating, compared with 43-23 percent in March.

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“The campaign has taken a toll on both candidates’ favorability ratings but more so on Kaminsky,” Greenberg said.

Laurence Hirsh, an accountant from Valley Stream running for the seat on the Green Party line, was not included in the poll.

In TV ads, mailers and meetings with district residents, McGrath has focused on the importance of keeping the Senate in Republican control, arguing that a Democratic win would allow New York City Mayor Bill de Blasio to direct school and tax aid from Long Island.

The special election could help determine the balance of power in the Senate. Republicans currently hold 31 of the 63 seats, but they control the chamber thanks to a governing coalition with six breakaway Democrats. There is one vacancy.

The poll showed that 51 percent of the Senate district residents want Republicans to maintain control of the Senate, while 42 percent want Democrats to take control of the chamber.

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Kaminsky has spent much of the campaign focused on ethics reform and has tried to link McGrath to Skelos, who was convicted with his son, Adam, in December on eight federal counts of bribery, extortion and conspiracy.

Dean and Adam Skelos are appealing, but on Friday a federal judge who will sentence them on April 28 refused to grant them a new trial after concluding that evidence produced by the government supported the jury verdict.