Whether Lindey Graham is gay or not, he's certainly screwing John McCain (And why immigration is next)
McCain is in a tough, but winnable, primary against crazy legend JD Hayworth, but something like immigration where McCain has a clear record against his party in a state where immigration is a top issue could rock that tough, but winnable primary.
(It's not clear whether the Dems actually want to be locked down on votes on an immigration bill before the midterms, but it is pretty clear they want the Republican opposition and tone on the record.)
Politically, I see three goals to the Democrats bringing up immigration. 1) An immigration debate will likely help Hispanic turnout and Hispanic Dem support in the immediate midterms.
2) With the current level of Republican crazy, this is just an invitation for them to (inevitably) cross the racist line which will not play well at all among independent voters.
Plus, blatant images of racism will also reflect and undermine all the previous "tea party" type opposition to Obama, playing into the narrative that the all white tea partiers are in fact holding racial grievances.
3) The inevitable Republican reaction evoked during this hyper partisan time will serve to solidify Hispanic support for Dems for the longer term. As Hispanics are the fastest growing voting block, that will really matter. (If Dems can even partially lock up a generation of Hispanics over racism the way they locked up African Americans....)