Republicans will gain seats in the 2018 midterm election.  The eight Republican seats up for grabs are easily defended, while Democrats will have five senators running for reelection in very red states: West Virginia, Indiana, North Dakota, Montana, and Missouri.  A pickup of five Senate seats is easy to see, and a gain of two or three is almost certain.

But Republicans could do much better than that if Trump is popular two years from now and if the Republican Party is united.  Eight other Democrat Senate seats are in states where Republicans have proven surprisingly strong in recent elections:  Florida, Ohio, Pennsylvania, Michigan, Wisconsin, Virginia, Minnesota, and New Mexico.

Florida is a Republican state.  It has a Republican governor and Republican legislature and just re-elected a Republican senator.  The House delegation is overwhelmingly Republican, and all the statewide elected officials are Republicans.

Ohio has almost exactly the same sort of structural advantages for Republicans in 2018.  The governor, legislature, House delegation, and statewide elected officials are Republican, and Trump carried the state easily.  Senator Portman won re-election by a landslide in 2016, and if John Kasich wanted to be in the Senate and re-enter national politics, he would be the favorite against Sherrod Brown, who barely won in 2012 when Obama was on the ballot.

Pennsylvania surprised the pundits by going for Trump, but anyone who looked at the composition of the Pennsylvania Legislature or the secondary statewide elective officers or the House delegation could see that Pennsylvania is more a Republican state than a Democrat state.  Senator Pat Toomey won in 2016 even with a Libertarian candidate winning over 3% of the vote.  This is a state, like other Rust Belt states, where if Trump brings good jobs back, Republicans ought to get help in 2018.   

Michigan has the same sort of configuration as Pennsylvania and Ohio – Republican legislature, Republican House delegation, Republican preponderance in secondary statewide offices, and Trump carried the state in 2016.  The state is changing.  This onetime union stronghold is now a right to work state.  If Trump can bring jobs to the Rust Belt, a good Republican can win the Senate race in Michigan.

Wisconsin has Republican control of nearly everything in state government.  Trump carried the state, and Senator Ron Johnson not only won a re-election he was thought likely to lose, but won easily.   The chief of staff for President Trump and the speaker of the House are from Wisconsin.  Tammy Baldwin, in an Obama election year, won only 51% of the vote.   She is a first-term senator from the minority party.  Scott Walker, with some powerful allies, could easily beat her.

Virginia is another state Republicans could win.  It is a state that is not strongly Democrat or Republican nationally – Democrat presidential candidates have carried Virginia only three times since 1972.  The Virginia Legislature is Republican, and if Republicans can capture the governorship in November 2017, then they may well be able to defeat Tim Kaine, a first-term minority senator like Baldwin. 

Minnesota might seem an odd place for Republicans to defeat an incumbent Democrat, but like Michigan, Pennsylvania, Ohio, and Wisconsin, Minnesota is becoming much more Republican.  Donald Trump came very close to carrying Minnesota, and Republicans captured the Minnesota Senate (giving them control of both houses of the legislature) and gained seats in the Minnesota House as well.  Two-term governor Tim Pawlenty would be a strong Senate candidate.

New Mexico was lost by Trump, but Hillary got less than half the vote.  Republican Governor Susana Martinez won her election in 2010 comfortably and won re-election in 2014.  She is term-limited and cannot run for re-election in 2018.  She could easily win the Senate race as a Hispanic woman in a state that is not strongly Democrat.

Republicans are unlikely to actually win 13 seats, but they could easily – in a good year – win 8 or 9, giving them a filibuster-proof majority and control, probably, for the whole of Trump's two terms, should he win re-election.  That means, as a practical matter, that President Trump and the Republican Party will have the chance to remake America for the better, if they have the guts.