[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/08/04/art.corzinead.cnn.jpg caption="The new Corzine spot features Obama."](CNN) - A hot summer in New Jersey appears to be getting even hotter for Gov. Jon Corzine, as a new poll suggests that Republican challenger Chris Christie has increased his edge over the incumbent Democrat in state's gubernatorial battle.
Fifty percent of likely New Jersey voters questioned in a Monmouth University/Gannett New Jersey survey released Tuesday back the former federal prosecutor in this year's election, with 36 percent supporting the first-term governor and 4 percent indicating they'll vote for independent candidate Christopher Daggett. Eight percent of likely voters are undecided. Christie's 14 point lead among likely voters is up from an 8 point lead in a Monmouth University poll conducted a month ago.
But among registered voters, including both those likely and unlikely to cast ballots in the gubernatorial contest, the survey indicates Christie holds a 4 point lead over Corzine, 43 percent to 39 percent, with 10 percent undecided. That's down from a 6 point lead a month ago.
The new poll was conducted after both President Barack Obama teamed up at a campaign rally with Corzine and the arrests by federal prosecutors of dozens of public officials and political operatives in New Jersey.
"We find that Corzine has made some gains with black and Hispanic voters, while Christie has increased his vote share among union workers. But the bottom line is that Christie's supporters are more engaged, which is why the Republican's lead among likely voters has grown," says Patrick Murray, director of the Monmouth University Polling Institute.
The survey suggests Corzine is taking three out of four Democratic voters, while Christie is winning eight out of ten Republicans and leading among independents by 20 points.
The 14 point lead for Christie among likely voters in the Monmouth poll follows nine point lead in Quinnipiac University survey conducted in mid July.
The new poll's release comes as the Corzine campaign is airing a new television commercial on broadcast and cable TV in New Jersey that features President Obama.
In the spot, using clips from last month's Corzine-Obama rally, the president says: "Jon Corzine wasn't just the first governor to pass an economic recovery plan for his state - he was an ally with the Obama administration in helping us develop a national recovery plan."
Corzine is seen standing next to Obama in the commercial, but doesn't speak. The Christie campaign dismissed the ad, noting that Corzine doesn't speak in the spot. "Considering nearly 200,000 New Jerseyans have lost their job in the last year, unemployment keeps rising and we still have the highest tax burden in the country, it's no surprise Jon Corzine is at a loss for words," says Christie Campaign Manager Bill Stepien
The Monmouth University/Gannett New Jersey poll was conducted 7/29-8/2, with 723 New Jersey registered voters questioned by telephone. The survey's sampling error is plus or minus 3.7 percentage points for registered voters and 4.5 percentage points for likely voters.