WASHINGTON - President Barack Obama marked 20 years of marriage to his wife Michelle on Wednesday with a romantic tweet sent 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 the president from his @BarackObama Twitter account.

The Obamas were married exactly 20 years ago Wednesday, on October 3, 1992.

One hour later, the first lady tweeted her response from her own account, #MichelleObama - a messages that she shared with her 1.69 million followers.

"Happy 20th anniversary, Barack. Thank you for being an incredible partner, friend, and father every day. I love you! - mo," she wrote.

Dinner by candlelight was not in the cards for the first couple, however, since their anniversary falls on the same day as the critically important first debate in Colorado between Obama and his Republican challenger Mitt Romney.

The showdown comes just 33 days before US voters go the polls on November 6, and will be held at the University of Denver in the western state of Colorado.

Obama has indicated that the first couple will push back their wedding anniversary celebration until the weekend.

Four hours ahead of the debate, the Democratic campaign team uploaded a 3.5-minute video on YouTube in which Michelle appeared praising her husband.

"I'm proud of Barack not just because he's a wonderful president but because he's maintained a solid foundation for our family," the US first lady says, as pictures of their daughters Malia, 14, and Sasha, 11, appear on the screen.

The clip includes segments of Michelle Obama's September speech at the Democratic National Convention praising her husband.