The 27-year-old has been a Lions target for a number of seasons and is on their shortlist once again this summer.
Boss Kenny Jackett was reported to have made a £300,000 bid for the defender in July 2011.
Jackett is searching for defensive reinforcements and has made a fresh move for Lewington after watching him in the Dons' 2-0 defeat to Huddersfield in the first leg of their League One play-off semi-final.
But Hornets boss and former Millwall centre back Sean Dyche has also made an approach.
Lewington has made nearly 400 career appearances and has been the Dons' captain for the past four seasons.