Larry Sabato's latest is up at Politico and in it, he analyzes the various scenarios that could lead to a GOP wave in November.

So where is the Senate right now? Hypothetically, there’s still a wide range of potential outcomes – and all the likeliest ones involve at least some kind of Republican wave, perhaps delivering the magic six-seat gain that would dethrone Democratic Senate Majority Leader Harry Reid.

Let’s adopt a mariner’s, or a meteorologist’s, terminology to make sense of the situation. Democrats hope for no wave at all: Something akin to “beach week,” the post-finals getaway that some of my University of Virginia students are celebrating this very instant on the Atlantic seaboard. Beach week for the Democrats, though, is about as likely as the undergrads’ dream of putting off adulthood. More realistically, Democrats can hope for fearsome-sounding waves that crash loudly but do little structural damage. Republicans, on the other hand, are rooting for an impressive tidal wave, if not a full-fledged historic tsunami.

The following table offers a wide range of Senate outcomes, along with their likelihoods.

Some of these scenarios have virtually no chance of happening, particularly the ones where Democrats play even in the Senate or even gain a seat. The Democrats have only a couple real chances to play offense, and their odds in either case are not great.

[...]

Meanwhile, there are 14 Democratic seats that are at least marginally practical targets for Republicans. The bigger the wave, the greater the number that could sink come Election Day.

According to the chart, much will depend on President Obama's approval ratings come November. Rich Baehr and others don't see the president's numbers recovering very much given what's ahead with more Obamacare surprises and a still moribund economy.