A month away from the first Republican primary debate, Governor Christie is taking a shot at President Trump over his reluctance to appear on the debate stage along with other GOP candidates, calling him a “chicken” in a new ad.

The former president has refused to commit to appearing on the debate stage at Milwaukee on August 23, saying it’s not the sort of thing a front runner in his position needs to do.

“Ronald Reagan didn’t do it, and a lot of other people didn’t do it. When you have a big lead, you don’t do it,” Mr. Trump told Fox News last week.

While Mr. Trump has plenty of time to change his mind — the deadline for committing to participate is just 48 hours before the debate — his appearance, or failure to appear, has major implications on the importance of the event. 

Now, Mr. Christie is calling out Mr. Trump for his refusal to commit to a debate in a new ad put out by a Christie-aligned PAC, Tell it Like It Is.

“If you don’t go you’ll be called a coward, a chicken, reduced to throwing spitballs from the sidelines,” the ad says. “So Donald, you need to decide, are you a chicken or just a loser?”

The ad is probably one of the most confrontational put out by anyone in the GOP field against Mr. Trump so far this year. It also attacks Mr. Trump for unfulfilled campaign promises.

“Should you show up to the debate?” the ad says. “If you do, your opponents will bring up the impeachments, the indictments, bring up how you lost to Joe Biden, lost the House, lost the Senate.”

Messrs. Trump and Christie, along with Governor Haley, businessman Vivek Ramaswamy, and Senator Scott, have all qualified for next month’s debate, and Mr. Trump is the only one who has indicated he will not attend.

There are other candidates, like Vice President Pence and Governor Burgum, who have indicated that they think they will meet the Republican National Committee’s requirements for appearing at the debate ahead of the deadline but have not yet done so.

To participate in the debate, candidates must poll above 1 percent in three independent national polls or two national polls and one poll of an early primary state. Candidates must also have more than 40,000 individual donors and need at least 200 donors in 20 states in order to qualify, a requirement that has been a hang-up for Mr. Pence.

“We make the polling requirement very easily,” Mr. Pence told CNN. “But yeah, having 40,000 individual donors, we’re literally working around the clock … I’m confident that we’ll be there.”

Some candidates, like Mr. Burgum, have resorted to gimmicks in an attempt to reach the 40,000-donor threshold. Mr. Burgum, who has been running a self-funded campaign, promised $20 gift cards to 50,000 donors.

The final requirement to appear on the stage is a pledge of fealty to the eventual Republican nominee in the general election. It’s not clear how seriously candidates are taking the pledge.

“I’m going to take the pledge just as seriously as Donald Trump took it in 2016,” Mr. Christie told CNN shortly after announcing his candidacy.

Whoever else appears on the stage, the importance of the event will be diminished if Mr. Trump follows through on skipping the debate, as he remains far and away the front runner, and the debate would be a prime opportunity for candidates to confront Mr. Trump.

This opportunity is perhaps more important for Mr. Christie than for any of the other candidates because the former New Jersey governor has centered his campaign around attacking Mr. Trump in a way that no other candidate has.