Lt. Gov. Barbara O'Brien is in charge when Gov. Bill Ritter is out of state.

That's what Colorado law says. But that's not good enough for the United States Senate.

Senate parliamentarians insisted that Ritter sign the certificate that lets Denver Public Schools Superintendent Michael Bennet take his seat, appointed by Ritter to replace Sen. Ken Salazar, the new Interior secretary.

And they insisted that Ritter sign the certificate on Colorado soil.

"The Senate is very strict in this matter," Ritter said.

So he flew home earlier than expected Wednesday from inauguration festivities. He signed four identical copies of the certificate at Denver International Airport minutes after getting off the plane.

The certificates, each with a big gold seal at the bottom, will be sent by overnight mail to the Senate. Bennet will be sworn in at 4 p.m. today in the Old Senate Chamber at the U.S. Capitol.

Ritter said he was sending three certificates to different Senate offices to make sure one gets into the right hands. The fourth went to Bennet himself.

Ritter used a different pen for each copy of the certificate. The four pens will go to Bennet's wife and each of his three daughters.

Bennet's family plans to stay in Denver, a spokesman said. Two of his children attend Denver schools. The third has not yet reached school age.