HARRIS CAMPAIGN DEMANDING A NEW CHANGE TO THE RULE ABOUT MUTING MICROPHONES. HERE'S GARRETT HAAKE. >> Reporter: TONIGHT THE TRUMP AND HARRIS CAMPAIGNS DUELING OVER NEXT MONTH'S CRITICAL PRESIDENTIAL DEBATE. THIS MORNING TRUMP CASTING DOUBT ABOUT ATTENDING. AND NOW A NEW WRINKLE. >> THE DEAL WAS WE KEEP THE SAME RULES. NOW ALL OF A SUDDEN THEY WANT TO MAKE A CHANGE IN THE RULES BECAUSE SHE CAN'T ANSWER QUESTIONS. >> Reporter: IN THE LAST FACE-OFF AT THE BIDEN CAMPAIGN'S REQUEST CANDIDATES' MICROPHONES WERE MUTED WHILE THE OTHER CANDIDATE WAS SPEAKING TO PREVENT INTERRUPTION. BUT NOW THE HARRIS CAMPAIGN IS ASKING FOR THE MICS TO BE LEFT ON AT ALL TIMES. TRUMP'S CAMPAIGN OBJECTING. BUT THE CANDIDATE SEEMING TO ACQUIESCE TODAY WHILE TAUNTING THE VICE PRESIDENT. >> DOESN'T MATTER TO ME. I'D RATHER HAVE IT PROBABLY ON. THE TRUTH IS THEY'RE TRYING TO GET OUT OF IT BECAUSE SHE DOESN'T WANT TO DEBATE. SHE'S NOT A GOOD DEBATER. SHE'S NOT A SMART PERSON. >> Reporter: VICE PRESIDENT HARRIS HAS BEEN OFF THE TRAIL SINCE HER ACCEPTANCE SPEECH LAST THURSDAY. SOURCES TELL NBC NEWS SHE HELD AT LEAST ONE MOCK DEBATE. >> WE'VE GOT 75 DAYS TO GO. SO MAYBE FOR BETTER AND FOR WORSE THAT'S THE WAY I AM, LIKE THAT WAS GOOD AND NOW WE'VE GOT TO MOVE ON. >> Reporter: ALL OF IT AS FORMER PRESIDENT TRUMP WAS IN BATTLEGROUND MICHIGAN LOOKING TO STEP ON ANY HARRIS POST-CONVENTION BOUNCE. CAMPAIGN SOURCES TELL NBC NEWS HE'S PLANNING MORE AGGRESSIVE FALL TRAVEL. AND WHILE POLLS SHOW VOTERS GIVE HARRIS THE ADVANTAGE ON HEALTH CARE AND ABORTION RIGHTS, TRUMP AIMING TO HAMMER HARRIS ON TWO ISSUES WHERE THEY SHOW SHE'S VULNERABLE, THE ECONOMY AND THE BORDER. >> UNDER KAMALA HARRIS AMERICAN VETERANS ARE TREATED WORSE BY FAR THAN ILLEGAL ALIENS. YOU EVER SEE THEM? THEY STAY AT HOTELS. WE HAVE VETERANS SLEEPING ON THE STREETS. >> Reporter: SPEAKING TO A NATIONAL GUARD CONFERENCE TRUMP SLAMMING THE VICE PRESIDENT OVER THE CHAOTIC WITHDRAWAL FROM AFGHANISTAN, WHERE 13 U.S. SERVICE MEMBERS WERE KILLED THREE YEARS AGO. TODAY TRUMP LAYING A WREATH AT ARLINGTON. >> THE HUMILIATION IN AFGHANISTAN SET OFF THE COLLAPSE OF AMERICAN CREDIBILITY AND RESPECT ALL AROUND THE WORLD. >> Reporter: HARRIS TODAY WRITING OF THE FALLEN, "MY PRAYERS ARE WITH THEIR FAMILIES AND LOVED ONES. MY HEART BREAKS FOR THEIR PAIN AND THEIR LOSS." >> GARRETT JOINS US NOW LIVE. GARRETT, LET'S GO BACK TO THE DEBATE FOR A MOMENT. THE HARRIS CAMPAIGN IS NOW RESPONDING TO TRUMP? >> Reporter: THAT'S RIGHT, TOM. A HARRIS CAMPAIGN AIDE SAYS THE FORMER PRESIDENT SHOULD HONOR HIS COMMITMENT AND AFTER TRUMP EARLIER SUGGESTED HE'S FINE WITHOUT THOSE MUTED MICS THE HARRIS CAMPAIGN I