A Democratic super PAC is reserving more than $20 million in TV, digital, and streaming ads in New Jersey in an effort to tamp down Republican inroads in the state during and since the 2024 election and to keep Democratic control of the state’s governorship.

Congresswoman Mikie Sherrill hopes to replace Governor Murphy, who was blocked from running again due to term limits. She is running against Jack Ciattarelli, who is backed by President Trump. Mr. Ciattarelli lost to Mr. Murphy by three points in the last gubernatorial election.

A group backed by the Democratic Governors Association, Greater Garden State, says it is reserving the ad buys early to lock in lower prices and reserve prime ad inventory before airwaves get crowded closer to the November election.

The major TV stations that cover about half of New Jersey are based in New York City, where a hotly contested mayoral race is expected to also vie for campaign ad time. New York is the most expensive local advertising market in the country.

“The DGA is committed to holding New Jersey’s governorship and making sure that voters up and down the Garden State understand the stakes of this election,” the political director of the group, Jordy Zeigler, says.

Ad spending could shatter records. More than $85 million was already spent in New Jersey’s gubernatorial primary, making it the most expensive election on record in state history, according to tracking firm AdImpact.

A Rutgers poll released last week shows Ms. Sherrill leading Mr. Ciattarelli 51 to 31 percent with 18 percent voting for someone else or unsure of who they are voting for. Analysts expect the race to tighten as the fall campaign heats up.

“Early polling on the governor’s race should serve as a baseline or a barometer of how voters are feeling in the moment — not as some crystal ball predicting the future four months from now,” the director of the Eagleton Center for Public Interest Polling at Rutgers University-New Brunswick, Ashley Koning, says. “A lot can happen between now and November.”

Mr. Trump looms large over the race. According to the poll, 52 percent of voters say he is a “major factor” in their vote for governor. Mr. Trump was mentioned in 70 percent of all broadcast ads during the primary, according to AdImpact.

Mr. Trump lost the Garden State by 6 points in 2024 after losing by 16 points in 2020. Republicans are hopeful their momentum will continue and they can pick up a governor’s seat.

The narrow vote margin in the last presidential election has some analysts now referring to New Jersey as a possible swing state after decades as a solidly blue state.

The number of Republicans registered to vote in the Garden State has surged in the past year and the voter edge for Democrats has dropped below 1 million, according to politics site New Jersey Globe.

A purge of the state’s voter rolls earlier this year also shrunk the gap between registered Republicans and Democrats. The state dropped nearly 225,000 voters who had missed multiple federal elections, with the Democratic count dropping by 50,000 more than Republicans.

The Republican Governor’s Association declined to share any figures about planned ad spending to support Mr. Ciattarelli, but a spokeswoman tells the Sun that the large ad buy shows Democrats are nervous about the race.

“DGA knows that Mikie Sherrill is a disaster of a candidate who isn’t offering a single solution to the problems voters in New Jersey are facing under one-party Democrat leadership,” the group’s communications director, Courtney Alexander, says. “Sherrill can’t stand on her own ideas and apparently can’t stand on her own campaign, so the DGA is already coming to bail her out.”

Virginia is the only other state with a gubernatorial election this year. The two elections are being watched as a possible barometer leading into next year’s mid-term elections.

Early polling in Virginia shows a former Democratic congresswoman, Abigail Spanberger, with a five-point lead over Republican Winsome Earle-Sears. If Ms. Spanberger wins, it would flip the office currently held by Governor Youngkin, who is not eligible for re-election due to Virginia blocking governors from serving consecutive terms.