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Thank you for the feedback.
Removing the Serial.begin() call was the trick. This function is not compatible with WatchDog Timer calls due to its excessive delay.
The code worked well without this...

I forget to mention that I commented out the SIM_COPC line in the startup_early_hook() as shown:

void startup_early_hook(void) {
// SIM_COPC = 0x0C; // 1024 ms
//SIM_COPC = 0x08; // 256 ms...

I added the watchdog start after the Serial.begin() as follows:
Serial.begin(57600);
SIM_COPC = 0x0C;

and the uP continuously resets if I don't open the "Serial Monitor" window. The...

I created a simple Watchdog timer program but Serial.begin interferes with its operation.
When I run with the "Serial Monitor" window open, it runs fine. When its closed, it does not.
I need it run...