Former Sen. Judd Gregg says that tonight’s presidential debate is “critical” for Republican challenger Mitt Romney.

“There are only three decision points for a challenger,” Gregg said Tuesday, “where he controls the events. One is when he picks his vice presidential nominee. Two is when he gives his acceptance speech, and three is the debates.

“And so the chance to speak in an unfiltered way to the American people about why Mitt Romney thinks he should be President and what are the big issues in this campaign,” Gregg said.

Gregg helped prepare former President George W. Bush for debates by “playing” Al Gore and John Kerry in pre-debate sessions in 2000 and 2004.

Romney “needs to get back to the big themes of this election,” Gregg said.

“This is a critical election and he needs to point out that the President wants to take us down the road to a European social welfare state and he wants us to continue on the path of being a country where we reward individual initiative and give people an opportunity to succeed,” said Gregg.

“He’s got to get off the minutia and onto the big themes.”

Gregg said it would be “foolish” for anyone to think the race is over.

“We all know that a week in political campaign is a light year,” he said. “Things change dramatically in politics. And this is a volatile year.”