The bid is said to be for a fee of around £8 million, a number that falls quite a bit short of the £12 million asking price for the Spurs defender.
However, Cardiff is hoping they will be able to work a deal for Caulker.
In an opposing report, though, David Hytner of the Guardian is saying that the Welsh club is seeking a season-long loan deal for Caulker.
The young English defender had a breakout season last year, making 18 league appearances for Spurs, notching his first two Premier League goals and even scoring for the England national team. Thanks to that fine season, Caulker has become a hot commodity.
However, it would be difficult for Spurs to handle losing Caulker from the center of their defense.
With the sale of William Gallas and the injury to Jan Vertonghen, the depth at centre-back is already being called into question. If Caulker were to exit White Hart Lane also, that would leave just captain Michael Dawson and Younes Kaboul, who is returning from a major injury, fully fit heading into the new season.
As a result, it makes little sense for Tottenham to part with their young commodity.
On the other hand, Declan Warrington of the Daily Mail is also reporting that Spurs are looking to sign 23-year-old Romanian defender Vlad Chiriches from Steaua Bucharest and the 20-year-old Portuguese centre-back Tiago Ilori from Sporting Lisbon.
If these signings do come to fruition, Tottenham might decide they can do without Caulker.
With the potential of each of those three options, it will be interesting to see how this transfer saga plays out.
However, it would be silly for Spurs to sell, or even loan, Steven Caulker unless they sign each of their two targets.