(CNN) Military chiefs have concerns about the politicization of President Donald Trump's July 4 event, a source with direct knowledge told CNN.
The US military is expected to be front and center during Thursday's event, which will showcase a wide variety of weaponry, including M1 Abrams tanks, and has taken on a political hue, with the President, who has already launched his re-election campaign, breaking tradition to deliver a speech and the Republican National Committee distributing tickets in a special VIP area.
Military leaders could be highly visible, as the service chiefs have been asked to stand with Trump during the celebrations, according to a different defense official.
If the President turns what is meant to be celebration into an overtly political event, he could put military personnel in the position of violating Defense Department guidelines prohibiting men and women in uniform from engaging in political activity.
In the planning for the event, Pentagon leaders also had reservations about putting tanks or other armored vehicles on display, the source said.
RELATED: Trump claims military is 'thrilled' to participate in his July 4 celebration
"Military members may not participate in political campaigns," said Jordan Libowitz, communications director for Citizens for Responsibility and Ethics in Washington. "This is likely the reason why they're hesitant to participate, because the guidelines are pretty clear about what they can and cannot do. If they come to stand next to him for a political speech while in uniform, representing the military, it could become a problem."
Libowitz points out that if the celebration becomes an overtly political event, there are also laws that come into play. A federal law known as the Hatch Act prohibits government employees and resources from being used for political activity and the Appropriations Act bars government funds from being used for purposes Congress hasn't approved, including publicity of propaganda.
The military chief's private concerns contrast sharply with Trump's claim Tuesday that "the Pentagon and our great Military Leaders are thrilled" to be participating in his revamped Fourth of July celebrations, which will also feature "incredible flyovers," fireworks and a speech he'll deliver.
The Chairman of the Joint Chiefs, Marine Corps Gen. Joseph Dunford, will attend. But Gen. Joseph Lengyel, the chief of the National Guard Bureau, is on an overseas USO tour that was planned since May and Gen. David Goldfein, the chief of staff of the Air Force, is on leave.
Air Force Col. Patrick Ryder, a spokesman for Dunford, said "the President extended an invitation to the Joint Chiefs and Gen. Dunford to attend the Salute to America event in Washington DC on July 4, which Gen. Dunford has accepted."
As the final details come together, several chiefs of the individual services are not attending and instead are sending alternates in their place, though some say they had prior plans.
Thomas Crosson, a Pentagon spokesman, said "it is not unusual for senior leaders to send a representative to functions that they cannot attend due to competing commitments. It is not in a military leader's DNA to task a subordinate to do something that the leader does not want to do."
A US defense official told CNN the White House also provided 5,000 tickets to the Pentagon.
This is not the first time military leaders have signaled their discomfort with potential politicization of the military in recent weeks. In early June, former acting Defense Secretary Patrick Shanahan issued an internal memo to all Pentagon personnel, service members and civilian employees calling on leaders to "reinforce the apolitical nature" of the US military, according to a copy obtained by CNN.
The memo was in response followed reports that the White House Military Office coordinated directly with the Navy's Seventh Fleet to have the USS John S. McCain hidden from view during Trump's visit to Japan. The late senator and Trump sparred often.
Several types of military aircraft are expected to take part in the event, including the new Marine One Presidential VH-92 helicopter, which will be making its debut, a defense official confirmed to CNN.
On Wednesday, Trump tweeted that "our July 4th Salute to America at the Lincoln Memorial is looking to be really big. It will be the show of a lifetime!"
He continued, stressing themes that are central to his re-election campaign and that could feature in the speech he's expected to deliver from the Lincoln Memorial. "We have the greatest economy anywhere in the world. We have the greatest military anywhere in the world. Not bad!"
Trump's plans for the event reverse decades of tradition in which the national holiday has been an apolitical affair in which the president has not been involved.
Not only is Trump involved, he has taken over planning and has reportedly been intensely involved in the details, including moving the fireworks from their usual location and expanding them to provide the "biggest ever Fireworks!" he declared in a Tuesday tweet.
Amid criticism of the extra cost to taxpayers to hold the event, Trump also tweeted that "the cost of our great Salute to America tomorrow will be very little compared to what it is worth."
He went on to say that "we own the planes, we have the pilots, the airport is right next door... all we need is the fuel. We own the tanks and all. Fireworks are donated."
But the President's tweet was misleading on a number of fronts, beginning with the fact that many of the planes will not come from nearby Andrews Air Force Base, but from around the country, according to a defense official.
That official said Navy F-35Cs will be coming from Naval Air Station Lemoore in California, Apache helicopters from Fort Campbell in Kentucky; Blue Angels F/A-18s will fly in from the Naval Air Station in Pensacola, Florida; and a B-2 bomber will be brought from Whiteman Air Force Base in Missouri.
Other aircraft, including F-22s, VC-25, Ospreys, and two of the F/A-18s, will come from Virginia and Maryland, the official said.
The President's tweet also ignores the additional costs of security, personnel and infrastructure required to put on an event of the size he wants. The clear ballistic armor necessary to protect Trump as he speaks costs $24,000 and the addition of extra metal detectors, Secret Service personnel and barriers necessary for him to speak will ratchet the cost up higher than in previous years.
That is apart from the high costs of the military demonstrations.
The National Park Service will redirect nearly $2.5 million to help cover costs related to the extravaganza, money that is usually "primarily intended to improve parks across the country," The Washington Post reported Tuesday.
The paper, which said administration officials "have consistently refused to say how much" the event will cost, noted that former National Park Service deputy director Denis P. Galvin said that typically, the July 4 celebration on the mall comes at a cost of about $2 million for the agency.
According to the Post, the $2.5 million redirected by the park service "represent just a fraction of the extra costs the government faces" for the event.
Here is the list of attendees as provided by a senior US defense official:
Acting Secretary of Defense Mark Esper
Gen. Joseph Dunford, Chairman of the Joint Chiefs of Staff
Secretary of the Navy, Richard Spencer
Adm. Bob Burke, Vice Chief of Naval Operations
Lt. Gen. Eric Smith, Deputy Commandant of the US Marine Corps
Acting Air Force Secretary Matthew Donovan
Gen. Stephen Wilson, Vice Chief of Staff of the Air Force
Adm. Karl Schultz, US Coast Guard Commandant
Mr. James McPherson, performing the Duties of the Under Secretary of the Army
Lt. Gen. Joseph P. Martin, incoming Vice Chief of Staff of the Army