As progressives gear up to contest the 2018 midterm Congressional elections, we will hear strategic, tactical, and political arguments for focusing on taking back the House or the Senate as the highest and best use of limited resources: money, volunteers, advertising, etc.. Progressives and other Democrats will want to recapture both chambers, especially after encouraging election wins in Alabama and elsewhere (mostly in state legislatures so far, as in Virginia, Oklahoma, Kansas, and Georgia). Nevertheless, it's fair to expect arguments about where best to put limited resources on the ground.
At the threshold, it's noteworthy that it's too early to judge whether the resources for the midterm campaigns will be limited so that some kind of triage is necessary. The 2017 Virginia state-government results - like the 2010 Tea Party wave - show that Blue voters can and will mobilize, organize, volunteer, and vote in unexpected numbers, as described here. The Jones victory in Alabama shows the same at the Congressional level, in a state with a long history of racist voter-suppression and a recent history of adopting an wholly unnecessary Voter-ID law (which had the unintended result of making it impossible for Republican Alabama government officials to credit any of Roy Moore's groundless allegations of voter-fraud).
Regardless what resources are available, it's at least as important to put a Democratic majority in the Senate as it is to put one in the House. In terms of Constitutional bang-for-the buck, it might even be more important to get a Democratic majority in the Senate.
The arguments for concentrating progressive resources on recapturing the House are straightforward and strong: Democrats need to add "only" 24 seats to their current 194 to take control of the House; Republicans currently hold 23 seats in districts carried by Hillary Clinton in 2016; Democrats are out-performing predictions in most special elections, even in special elections they have lost (e.g., Jon Ossoff's race in Georgia). Larry Sabato's Crysal Ball rates 16 R-held House seats as toss-ups, but only 4 D-held House seats as toss-ups.
In contrast, also according to Sabato, of the five toss-up races that could change control of the Senate, three are for seats held by Democrats in states Trump won - Missouri (McCaskill), Indiana (Donnelly) and Florida ( Nelson). Notwithstanding Jones's win in Alabama, 10 Senate Democrats are up for re-election in states Trump won in 2018.
It certainly appears that the House is the path of less resistance to a Democratic-majority-chamber, especially as Democrats are doing very well in the generic Congressional ballot, a strong predictor of control of the House. Per Nate Silver: "I’ve previously calculated that the Democrats need to win the national House vote by 5.5 to 8 points to win the House." Right now, Democrats are favored in the generic Congressional poll by almost 13%, though, as Silver notes, these numbers can and likely will change between now and Election Day 2018. But he adds that, "...the generic ballot at this point is usually predictive of the midterm House result."
That said, it's important for several reasons to protect Democratic Senate seats and try to take a majority in the Senate.
First, Mitch McConnell has been ruthless in his use of the Republican Senate majority. He's abandoned normal legislative processes for legislation and Senate confirmations of personnel for the executive and judicial branches. Instead of hearings, mark-ups, and measured deliberation on bills and nominees, McConnell's produced absurd and opaque circuses represented most notably by the failed direct repeal of the Affordable Care Act and the recent tax bill. Taking the leadership of the Senate away from McConnell would begin the restoration of normal legislative processes in the Senate - not always or only a virtue, but often more transparent and inclusive than anything we've seen during McConnell's Republican unilateralism.
Second, with the possibility of a blue wave in the House, why not contest the Senate? Mind, I don't base this suggestion on the argument that, "If Democrats can win in Alabama, then they can win anywhere!" Circumstances in Alabama were unusual this year - Republicans nominated Roy Moore, an appalling Steve-Bannon-approved theocrat (and worse).
However, if progressives and moderate Democrats are widely mobilized and put up attractive and competent candidates everywhere they can for House seats and state legislative seats, those candidates could have reverse-coattails up-ballot, for Senate seats. And Steve Bannon's efforts to remake the Republican party in his image could backfire as badly in other Senate races with Republican incumbents or open Republican-leaning seats (Nevada and Arizona, for example) as they did in Sweet Home Alabama.
Third, taking the Senate would be provide more Constitutional power for Democrats than taking the House. Sure, Democrats can block the Trump/Republican Gilded Age legislative agenda with control of either chamber, but it seems reasonable to expect a Democratic majority in the Senate would also brake or halt Senate confirmations of unqualified and destructive nominees to the executive branch. A Democratic majority also would control Senate committee investigations of the Trump administration. And - most importantly - a Democratic majority in the Senate would probably halt the appointment, to the federal judiciary, of lifetime-tenured incompetents, right-wingers, and right-wing incompetents.
Trump could continue to nominate the unworthy and unskilled, and doubtless will, but it's a near-certainty that a Democratic-majority Senate would not confirm them. Even this Republican-controlled Senate is finding a few of these hacks and nuts unconfirmable. It seems very likely that a Democratic majority on the Senate Judiciary Committee and in the Senate as a body would be far less willing to approve Trump judicial nominations.
In other words, if Democrats recapture the Senate, they can protect more than just their Constitutional branch from the depredations of Trump and his Republican enablers; they can protect the third branch - the judiciary - as well. With potential Supreme Court vacancies on the horizon (Justices Ginsberg and Kennedy, for example), this is a matter of high importance, especially as repressive state anti-abortion and redistricting laws will continue to make their way into and through the courts.
Therefore, as you decide where to put your volunteerism and dollars in 2018, please keep in mind that two Constitutional branches of our government - not just one - are at play in the midterm election. Ignoring Senate races can be extremely hazardous to our Constitutional health for a long time - a lifetime-tenured long time.