Barack Obama marked his 20th wedding anniversary by squaring off against rival Mitt Romney in their first debate - and First Lady Michelle was 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),'" she told CNN in an interview that aired just before the two candidates took to the podiums in Denver, Colorado, on Wednesday night. "I get so nervous at these debates," she said. "I would not have chosen this, but I'm excited about it." The 51-year-old president began the debate with a message to his wife, apologising for the unromantic setting. "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." Romney joked: "Congratulations to you, Mr. President, on your anniversary. I'm sure this is the most romantic place you could imagine, here with me." 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. Obama has said that he and Michelle will push back their anniversary celebrations until the weekend. Michelle Obama and Ann Romney were both on hand in Denver for the debate, and the two women shared a brief embrace shortly before it began.