Caulker joined Cardiff from Tottenham Hotspur last July for a then club record fee of £9 million, but despite some strong displays the centre back was to powerless to prevent the Welsh capital club being relegated from the Premier League.
The ex-Spurs trainee, 22, also enjoyed a successful loan spell at the Liberty Stadium in 2011-12, when he played 26 times for the Swans in their debut Premier League season. Caulker scored the only goal as Cardiff beat Swansea in the Premier League's first ever South Wales derby last November.
With Swans captain Ashley Williams being linked with a £3 million away from the Liberty, it is believed Swansea boss Garry Monk has targeted Caulker as a potential replacement.
Caulker is reported to have a relegation release clause in his contract, so Cardiff would be powerless in preventing their captain speaking to their rivals should a sufficient bid be made. Liverpool and Tottenham are also reputed to be interested in signing the once-capped England international, who is keen to remain in the Premier League next season.
A move to Swansea would cause controversy in South Wales and would add even more bitterness to what has been a miserable 2014 so far for the Bluebirds.
However, Cardiff will have to sell this summer to balance the books following relegation and if the club can recoup most of what was paid for Caulker it is unlikely much care will be given by the club's hierarchy about who pays. Chile midfielder Gary Medel is expected to leave, and reports have also linked goalkeeper David Marshall, striker Fraizer Campbell, and midfielder Jordon Mutch with moves back to the Premier League.
Former Cardiff City and Wales striker Nathan Blake has previously said Cardiff will be better off without Caulker next season, citing the captain's suspect defensive play.
Despite the acrimony between the clubs supporters there are a number of players who have successfully 'crossed the South Wales divide' and Caulker, having already done it anyway, wouldn't even be the first Cardiff player to transfer to Swansea.
However, if the deal does come off it will certainly be the most high profile Cardiff-Swansea transfer, and if it can be done fairly cheaply it will mark off another victory for the Swans over their rivals.
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