Although negotiations are at a preliminary stage and obstacles remain, the Nationals and Rangers have had trade discussions involving four-time All-Star Cole Hamels, sources told MLB.com Monday.

The Hamels talks illuminate one key aspect of the Nationals' approach to the July 31 non-waiver Trade Deadline: As long as they are viable contenders for a National League playoff spot, they will look to upgrade their starting rotation.

For the Rangers, Hamels' contract is an impediment in consummating a trade -- with the Nationals or any other club. He's owed more than $7 million over the balance of this season, followed by a $20 million club option (or $6 million buyout) for 2019.

The Rangers likely would be asked to assume a portion of Hamels' future salary obligations, particularly in light of his recent performance. Due in part to spotty fastball command, he has a 5.28 ERA since the start of June.

Still, it helps the Nationals in their effort to land Hamels that they are not among the 20 teams to which he can block a trade.

The Nationals' rotation long has been one of the franchise's greatest strengths, but that has changed this year. Nationals starters have combined for a 33-36 record and 4.05 ERA, which ranks 14th in the Majors. Last year, they were fourth with a 3.63 ERA.

The Mariners and Yankees have also shown interest in Hamels previously. The Braves have scouted Hamels as well in recent weeks and are monitoring his marketplace, sources said.