US President Barack Obama marked 20 years of marriage to his wife Michelle on Wedneady 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 on Wednesday, on October 3, 1992.

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 American voters go the polls on November 6, and will be held at the University of Denver in the western state of Colorado.

Obama said in an appearance on a US television talkshow last week that he and Michelle will push back their wedding anniversary celebration until the weekend.