Facing challengers from right (Tea Party) and left (a strong Democratic candidate) in his forthcoming reelection campaign, Mitch McConnell has problems no matter what positions he takes. In that circumstance, we should expect him, like most experienced, cynical politicians, to take whichever position losses him the fewest votes, all things considered. Here are two factors that might have led him to make the decision to do a deal with Harry Reid, rather than just sit tight: (1) He currently holds a 47-point lead in Kentucky public opinion polls over his Tea Party primary challenger; and (2) McConnell inserted into the short-term budget fix a provision to fund two new dams in Kentucky.
As to (1), with the midterm elections still a year away, McConnell's big lead over his Tea Party primary challenger could well evaporate, but he might well hope that broader voter support for Tea Party candidates might wane in light of their extremism; and he might come off looking like a statesman by comparison. No doubt, his staff did their own calculations on the likely costs and benefits of his doing a deal with Harry Reid, and decided that his current lead in the polls was unlikely to evaporate as a consequence.
As for (2), I find it repulsive that Mitch McConnell used his political leverage and the cover of national concerns at stake to sneak into law funding for two dams in his home state. That's about as venal as you can you get. But will Kentuckians see it that way? Or will they just see their stalwart Senator bringing home the bacon? If the later, it might well gain him some general election votes against his Democratic rival. At the very least, it is a reminder that as a (very) senior Senator, McConnell is better positioned to provide for his constituents than a brand new Senate (regardless of party affiliation) would be.