One of the complicating factors involved with trading for Ryan Dempster is that he has 10/5 rights, which means that he can veto any trade.
However, Dempster has reportedly told the Cubs that he would accept a trade to the Rangers, the Yankees or the Dodgers. Thus, if the Rangers wanted to trade for Dempster, the 10/5 rights would not be a hurdle.
That being said, it still seems unlikely the Rangers would trade for Dempster. He is a free agent after the season, and there's some question as to whether he's viewed as a significant upgrade over the pitchers the Rangers have in-house. The Cubs had an agreement to get Randall Delgado from the Braves in exchange for Dempster before Dempster vetoed that deal. That might suggest that the equivalent cost from the Rangers would be Martin Perez (although Perez is probably more valuable of a trade chip than Delgado right now).
In any case, the Rangers have shown a reluctance to move their prime prospects for a rental, and it seems unlikely they'd trade a top prospect for Dempster.