DENVER: US President Barack Obama is marking his 20th wedding anniversary by squaring off against rival Mitt Romney in their first debate and First Lady Michelle is less than thrilled.

"I told Barack, 'This, you know, attending a presidential debate on my 20th anniversary is probably the worst way for me to spend (it),'" Michelle Obama told CNN in an interview that aired shortly before the two candidates took to the podiums in Denver, Colorado.

"I get so nervous at these debates," she said. "I would not have chosen this, but I'm excited about it."

The president marked the anniversary by sending a romantic tweet to the first lady and to his 20 million followers on Twitter.

"Twenty years ago today, I married the love of my life and my best friend. Happy anniversary, Michelle. -bo" read the message sent by Obama from his @BarackObama Twitter account.

Later, as the high-stakes debate opened in Denver, Obama turned romantic once again.

"A lot of points I want to make tonight, but the most important one is that 20 years ago, I became the luckiest man on Earth because Michelle Obama agreed to marry me," the president said.

Then, turning to his wife, he said "I want to wish you happy anniversary, and a year from now, we won't celebrate it in front of 40 million people."

The Obamas were married on October 3, 1992. But with the critically important first debate taking place Wednesday, dinner by candlelight was not in the cards for the first couple.

The showdown comes just 33 days before American voters go the polls on November 6, and was being held at the University of Denver.

Obama has said that he and Michelle will push back their anniversary celebration until the weekend.

The first lady declined to offer a critique of her husband.

"I really would probably be the worst person to assess his style or his techniques," she told CNN.

"There are the rules, and you don't want to clap... So I'm just trying to make sure I'm following the rules."

Michelle Obama and Ann Romney were both on hand in Denver for the debate, and the two women shared a brief embrace shortly before the event began.

Romney said her husband has followed a specific routine at the start of each of his 20 debates during the Republican primary race, and she expects him to do the same with the presidential showdowns.

"As soon as he gets on stage... he takes off his watch and puts it on the podium," she told CNN.

"Then he writes 'Dad' on the piece of paper," in honor of his father George Romney, who was governor of Michigan and also ran for president in the 1960s.