If Democrats want to deliver a statement in 2018, the House would be the place to do it because they are almost certainly not going to take the Senate. Democrats will have to defend 23 seats, independents will have to defend 2, and the Republicans have to defend 9. In order to take control of the Senate, Democrats have to gain a net addition of three seats, since any scenario in which the GOP controls at least 50 seats would enable Pence to cast his VP-tiebreaking-vote in the event of a tie.
Hillary-State Democratic Senate seats up for contest (13): California, Maryland, Delaware, New York, Massachusetts, Connecticut, Minnesota, Washington, New Hampshire, New Mexico, Hawaii, New Jersey, Virginia
Trump-State (Vulnerable) Democratic Senate seats up for contest (10): Indiana, Montana, Missouri, West Virginia, South Dakota, Pennsylvania, Ohio, Michigan, Wisconsin, Florida
Independent Senate seats up for contest (2): Vermont, Maine
Hillary-State (Vulnerable) (1): Republican Senate seats up for contest: Nevada
Trump-State Republican Senate seats up for contest (8):
Utah, Arizona, Wyoming, Nebraska, Texas, Alabama, Mississippi, Tennessee
**2016 Election-Map Scenario: **Republicans win all the Senate races in Trump states and Democrats take all the Senate races in Hillary states (we’ll disregard independent Senators). In that scenario, the Republicans would gain 9 seats and take a 61-37-2 lead in the Senate. This scenario almost certainly wouldn’t unfold, though, as Trump and the GOP will likely be deeply unpopular by November 2018.
Each party takes the other side’s vulnerable seats only: Republicans capture the Democratic seats in South Dakota, Montana, Indiana, West Virginia and Missouri - all deep red states - and Democrats take the Republican seat in Nevada. All other seats remain in the same hands as before. In that scenario, Republicans gain 4 seats and take a 56-42-2 lead in the Senate.
Democratic-Wave Scenario: A tidal wave sweeps Democrats into victory across the board. It’s still highly unlikely the Democrats can take Texas, Wyoming, Utah, Nebraska, Mississippi or Alabama, but they could snag Nevada, Arizona, and hold all of their 23 blue seats. If they snatch an upset win in one of the above mentioned seats as well, this would mean Democrats gain a 51-49 advantage in the Senate(technically 49-49-2, but we could count the two independents as de facto Democrats.) But this would be an extraordinarily slim lead in the Senate. But this, again, requires the Democrats to hold on to each and every single one of their 23 Senate seats being contested, including the vulnerable five of MO-MT-SD-WV-IN, the deep red states.
Republican-Wave Scenario: *Extraordinarily *unlikely, but if there were some unexpected event like a major Islamic terrorist attack (that Democrats were perceived as abetting due to opposing a Muslim ban), or unlikely economic boom, or something that creates a rally-around-the-flag event, we could see momentum behind the GOP’s backs, not the Democrats, and this might enable Republicans to not only defend all nine of their seats, but also take the 5 most vulnerable Democratic seats and some other besides, potentially taking perhaps a 63-35-2 lead.
The only way the Democrats take the Senate in 2018 is if there is a massive tidal wave in their favor that allows them to defend every single one of their 23 seats, ***and ***independents don’t lose to GOP challengers, ***and ***the GOP loses three or more of its nine seats. But even then, that is only a Democratic “majority” by the slimmest of margins, vulnerable to defectors on key-issue votes. And the Democrats losing a race in just one or two of those 23 Blue seats would throw a wrench in all that.
And none of these above scenarios involve independents losing. Bernie Sanders is pretty much invincible in Vermont, assuming he runs again, but Angus King in Maine is vulnerable, although if the Republicans nominate LePage in Maine then they can kiss any prospects of snatching that Maine seat goodbye.
Possible outcome: Democrats take the House but the Republicans run up their lead in the Senate.