2018 US Senate Ranking from most likely to flip to least likely to flip.

[nkpolitics1212]

4. Regarding PA(Specter-D), Rendell was not Governor after 2010 due to term limits.

Tom Corbett-R was. Corbett a conservative Republican would have appointed a Republican to run in the 2014 special election and again in the 2016 regular election.
2020 will be the year Democrats will regain control of the US Senate. Unlike in 2018, None of the Democratic incumbents US Senators up for re-election in 2020 are from red states. MI (Peters-D),MN (Franken-D),NH (Shaheen-D) and VA (Warner-D) are from purple to weak blue states. I expect Democrats to hold onto MI,MN,NH,and VA in 2020 and pick up CO(Gardner-R) and NC (Tillis-R) in 2020. The 3rd seat Democrats will pick up in 2020 is AZ-special election to replace McCain-R. Democrats could also pick up MT(Daines-R)-if Bullock-D runs and ME if LePage-R decides to challenge Collins-R in the Republican primary and wins, Collins decides to run as an Independent.
In 2022-Democrats will hold onto AZ (Winner of the 2020 Special Election),CO (Bennet-D),NV (Cortez Masto-D),and NH (Hassan-D). Democrats could pick up FL (Rubio-R), IA (OPEN-Grassley-R), NC (Burr-R), PA (Toomey-R), and WI (Johnson-R)