A bill making it easier for prosecutors in Colorado to convict people of driving high on marijuana died in the state Senate special session today because one key supporter was absent.

After the Senate voted down the bill on an informal vote, an effort to revive it failed on a 17-17 split. The missing vote was that of Sen. Nancy Spence, a Centennial Republican who was the deciding vote on a nearly identical bill in the legislature’s regular session.

Reached by phone, Spence said she was in San Diego, where she had plans to celebrate her grandson’s birthday that were made well before the special session was called. Spence said she was prepared to fly back to Denver on short notice to vote for the bill but that she didn’t know the bill would be brought before the full Senate today.

“I’m really sad about it,” Spence said. “I feel terrible for (bill sponsor) Steve King, who worked so hard to get that bill passed.”

The result was surprising for a bill that, again, appeared headed for passage before being tripped up late. Earlier in the day, the Senate State, Veterans and Military Affairs Committee passed the bill 4-1. That came after the bill received its final approval in the House this morning.

That meant the bill needed to pass only two votes — one today and one tomorrow — by the full Senate to head to the governor’s desk. Gov. John Hickenlooper has said he supports the bill.

The bill, HB12S-1005, would make it a crime to drive with more than a certain amount of THC — the psychoactive chemical in marijuana — in your blood. A nearly identical proposal appeared headed for passage during this year’s regular legislative session. But it became entangled last week in the end-of-session fight over civil unions and died on the calendar.

Spence said she received a text message this morning from Senate Minority Leader Bill Cadman, R-Colorado Springs, asking her how soon she could return to Colorado. Spence said she asked how soon she was needed but that she didn’t know the Senate would be taking up the bill today until she turned on her laptop to watch the Senate session remotely.

“I assumed it wouldn’t be brought up until tomorrow morning,” Spence said.

“I’m just so, so sorry and so disappointed,” she added.

Cadman said he wasn’t sure Spence’s presence would have made a difference, saying that several lawmakers were keeping quiet about their vote and might have switched against the bill had Spence been there.

“That’s how close and, I think, complicated this issue became,” Cadman, who voted for the bill, said.

Reviewing the vote shows lawmakers voted today exactly as they had during the regular session.

Senate President Brandon Shaffer, a Longmont Democrat who also supported the bill, said lawmakers didn’t play games with the bill schedule. Because every day in a special session costs the state more than $23,000, Shaffer said legislative leadership wanted to keep the session to three days. That meant the bill had to be heard today in order to pass.

“We weren’t trying to pull a fast one,” Shaffer said.

Shaffer said Spence didn’t ask for permission to miss the vote. She was listed as absent for the vote, contrary to the usual courtesy of listing lawmakers who have to miss votes as excused.

“I was very disappointed with the outcome of today’s vote,” Shaffer said.

At the earlier committee hearing, medical-marijuana activists argue that the proposed limit — 5 nanograms of THC per milliliter of blood — is too low and would result in near-certain convictions for sober drivers.

The bill’s opponents argued that medical-marijuana patients have no way of determining what 5 nanograms means. How much can they consume? How long do they have to wait afterward?

“There needs to be a way to know whether a medical patient has the 5 nanograms in their system so they can know whether they can get behind the wheel,” said Debbie Olander, a representative from the United Food and Commercial Workers union. “With alcohol you can.”

Supporters of the bill counter that the vast majority of people would be impaired at 5 nanograms and would need to wait only about two to three hours after using to fall below the limit. They argue that, even though some people could be sober at 5 nanograms, it is important to send a strong message.

“The law works best when there are clear, effective, enforceable standards,” Boulder District Attorney Stan Garnett said, testifying in support of the bill.

The regular-session version of the bill passed the full Senate by a single vote.

John Ingold: 303-954-1068 or jingold@denverpost.com