Any public (or "government-run") option in healthcare reform legislation must be subject to a so-called "trigger" mechanism, one of the Senate's key centrist Democrats declared Sunday.

Sen. Ben Nelson (D-Neb.) said that he hoped President Obama would back a trigger for the public option when he addresses a joint session of Congress on Wednesday.

"I think he has to say that if there's going to be a public option, it's going to be subject to a trigger," Nelson said during an appearance on CNN this morning.

The "trigger" has been bandied about as a compromise to win over centrist Democrats and some Republicans on the public option. Under the plan, the establishment of a public option would be delayer, and only "triggered" into place if healthcare market conditions persist or deteriorate.

"I don't mean a hair trigger," Nelson added of his expectations of the plan. "I mean that one that would only apply if there isn't the kind of competition in business that we'd hope there would be."

Nelson argued that the trigger plan and the president's speech this week may be finally able to push negotiations on healthcare reform after an extended impasse over the August recess.

"That might break the logjam and bring some Republicans on," he said. "Perhaps with the right combination of ideas and a trigger and things like that, others might join the process."

Sen. Amy Klobuchar (D-Minn.), appearing alongside Nelson on CNN, said that Obama's speech on Wednesday night will be an opportunity.

"I think it's a great opportunity...for him to put the meat on the bones," she said. "I think they have an opportunity to grab the message now, and perfect the message."