From Andrew Prokop’s Vox.com post, “Why Democrats increasingly think Donald Trump can deliver them a Senate landslide“:

In addition to competing in a set of Senate contests that have long been expected to be in play, Democrats have managed to recruit well-credentialed and potentially formidable challengers in many “reach” Senate races where the party wouldn’t ordinarily expect a win.
These contenders are running mostly in red states, and most of them would likely lose in a “normal” presidential election year. But a backlash against Trump could potentially put these seats into play, so long as credible challengers were ready and waiting to take advantage of the situation.
“We wanted to have as many surfers on the water as we could, because we didn’t know how big the wave would be,” a Democratic strategist involved in Senate races recently told me.
Indeed, the surprise dynamics of the presidential race and the apparent strength of these contenders make the upper bound for Democratic gains very high indeed — north of 10 seats. With a strong performance across the board, Democrats could end up with 56 seats or even more overall, which is a very solid majority.

Prokop adds, “The upshot, of course, is that Democrats get to pick and choose which Republican-held seats to gun for, while playing defense in very few races.” He sees three basic categories of 2016 U.S. Senate races:

The top-tier GOP-held battlegrounds: These are six Republican-held seats that have long looked vulnerable — Obama managed to win all these states twice. In five of these, senators who first won their seats in 2010 will face a presidential-year electorate for the first time, while the other seat is open.
The “reach” GOP-held targets: Then there are another six GOP-held seats, mostly in redder states, that would likely be out of reach in a typical year. But Democrats think they’ve recruited strong challengers in all six who could be competitive in the Year of Trump.
The few Democratic defenses: Finally, there are a mere two Democratic-held seats that are being seriously contested by the GOP. And both are in states with growing Hispanic populations, so Democrats are hoping Trump’s rise will hurt Republicans badly here.

Here’s a map illustrating Democratic prospects for winning a senate majority at the current political moment, based on Prokop’s analysis:

Prokop has paragraphs analyzing each key senate race, and you can read more about it right here.