It’s quite possible writes Larry J. Sabato in Politico. The basic reason why is pretty simple:

In 2016, the Republicans seem to have their backs to the wall, defending 24 seats to the Democrats’ 10. Just two Democratic seats—Harry Reid’s in Nevada and Michael Bennet’s in Colorado—are not solid, both in states where the GOP surged in 2014.

And:

Meanwhile, Republicans must defend seven incumbents that represent states carried by President Obama in 2008 and 2012: Kelly Ayotte of New Hampshire, Chuck Grassley of Iowa, Ron Johnson of Wisconsin, Mark Kirk of Illinois, Rob Portman of Ohio, Marco Rubio of Florida, and Pat Toomey of Pennsylvania. An eighth state Obama carried in 2008 but narrowly lost in 2012—North Carolina, home to two-term Republican Richard Burr—also merits mention with these other states.

So long as Grassley runs for a seventh term—and he says that he will—it’s doubtful Democrats can credibly challenge him.

But under the right conditions—i.e., strong Democratic Senate candidates combined with a solid national lead for the Democratic presidential nominee—all seven of the other seats are vulnerable. Three of the states (Illinois, Pennsylvania, and Wisconsin) are more Democratic than the nation as a whole and have a history of voting Democratic for president even when the party loses a close race nationally, as in 2000 and 2004. This reality has placed Kirk, Toomey and Johnson at the top of the endangered senators list.