Mitt Romney has taken a six-point lead over President Obama in the latest Gallup national tracking poll -- his biggest lead to date and the first time he has led outside the margin of error.

The latest seven-day tracking poll of likely voters shows Romney at 51 percent and Obama at 45 percent, up from 50-46 on Tuesday and 49-47 on Monday.

Romney has steadily gained in the Gallup poll in recent weeks, turning what had been a growing deficit in September into a growing lead since his strong first debate performance. And when Gallup shifted its voter model from registered voters to likely voters last week, Romney's numbers improved even more (among registered voters, the race is at Romney 48, Obama 46).

The new numbers, of course, don't include much or any data collected after Tuesday night's debate. It will take days to determine what effect that might have had.

Gallup continues to show a better picture for Romney than most other pollsters. A Washington Post-ABC News poll released Monday showed Obama at 49 percent and Romney at 46 percent, while a Politico-George Washington University poll showed Obama at 49 percent and Romney at 48 percent.

In addition, Democrats have cast doubt on Gallup's likely-voter model, with the Obama campaign releasing an entire memo on it earlier this week when Gallup showed Romney opening up a similar lead in the 12 most competitive states in the country.