11 Safe Democratic seats: Colorado (Republican held), Delaware, Illinois, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New Mexico, Rhode Island, and Virginia.
17 Safe Republican seats: Arkansas, Alaska, Idaho, Iowa, Kansas, Kentucky, Louisiana, Maine (assuming Collins runs), Mississippi, Nebraska, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, West Virginia, and Wyoming.
Competitive seats: Alabama (if Jones runs), Georgia, North Carolina, and Montana (If Bullock runs). Due to increasing political polarization and the fact that it is a presidential year, 2020 will see very few competitive Senate races.
I consider competitive to mean that both parties have a real shot at winning. Colorado is not competitive because Cory Gardner will lose. He has no chance at winning reelection in a blue state in a Presidential year.
Iowa and Alaska seem like possible competitive seats, at least under the right conditions. The last two elections in this Alaska Senate seat were very close races, which at the very least goes to show a wave of sorts can flip them. If things keep going the way they are going, I think it's probable that the 2018-2020 period will be like a 2006-2008 redux, although with the Democratic Party probably being somewhat less liked.
Something I also pointed out to someone else was that if Democrats won back the Senate in 2018, and swept competitive races in 2020, they could actually end up holding the Senate through 2022 even if they won back the White House in 2020. The map in '22 isn't that favorable for Republicans, given their performance in the last elections for that class. And if Trump won a 2nd term, Democrats would probably be able to put the prospect of a supermajority on the table.
Collins is safe. She has not had a close race since she first ran for Senate in 1996.
Not having a close race before doesn't mean it will never happen. There are examples of longtime Senate/House incumbents winning easily up until a big wave swept them away, even if only narrowly - Such as Alaska 2008, iirc.
