The U.S. Capitol dome is seen in Washington. (Chip Somodevilla/Getty Images)

For months now, the focus of Campaign 2018, rightly, has been on control of the House. All the metrics continue to point to a midterm election in which Democrats could seize control of that chamber. But for sheer drama and unpredictability, the contest for control of the Senate could be the place to look.

The House is no slam-dunk for the Democrats, but most Republicans following the campaigns are genuinely worried and probably right to be that way. The overall environment is difficult for the GOP because of President Trump and because of the location of the competitive races; suburban areas as one example. There are so many Republican-held seats at risk (and very few Democratic seats in similar danger) that Democrats have multiple paths to pick up the 23 they need to flip the chamber.

The Senate is and has been a different story. There the Democrats’ prospects are much more difficult, in large part because of the two big structural differences with the battle for the House. If the terrain that will determine control of the House more generally reflects the breadth of the country, the campaign for the Senate is largely playing out in the heart of Trump country.

Republicans are only defending nine of the 35 Senate seats up in November. They have to play far less defense than the Democrats. Second, many of the most competitive Democratic-held seats are in states Trump won easily in 2016: West Virginia by 42 points; North Dakota by 36 points; Montana by 20 points; Indiana and Missouri each by 19 points.

The range of possibilities in the Senate is not at all the same as in the House. No one questions whether Democrats will gain seats in the House in two months. The question is how many: a few short of the 23 they need, a few more than 23 or a lot more than 23. In the Senate, Republicans could, narrowly, lose control of the chamber or they could end up bolstering their slender two-seat majority.

The state of the races offers few definitive clues as to what is coming. Almost everywhere you look the contests are tight. Florida features one of the premier Senate races of the cycle and probably the costliest, pitting Democratic incumbent Bill Nelson against Rick Scott, the term-limited Republican governor.

Florida is the perennial swing state: the purple monster of American politics. So maybe it’s no surprise that things are close. (Trump won the state by a percentage point in 2016. Barack Obama carried it by three points in 2008 and then by just a point in his 2012 reelection. George W. Bush won it by 537 disputed votes in 2000 and then delivered what amounts to a landslide in the vernacular of Florida presidential results, a five-point victory in 2004.)

A recent Quinnipiac University poll shows Nelson and Scott tied at 49 percent, as have some other relatively recent polls. Scott is used to this. In two elections for governor, his victory margins were about a percentage point each time. Nelson won an easy reelection six years ago by double digits, but those days are gone.

Florida isn’t the only state where things look tight. An NBC-Marist poll of Missouri shows Democratic Sen. Claire McCaskill and state Attorney General Josh Hawley even at 47 percent each. In Tennessee, NBC-Marist shows a statistical dead heat between former Democratic governor Phil Bredesen and Republican Rep. Marsha Blackburn (48 percent for Bredesen, 46 percent for Blackburn). In Nevada, where there hasn’t been much recent polling, Republican Sen. Dean Heller and Democratic Rep. Jacky Rosen are also judged to be in a race that is a virtual toss-up.

You get the idea. Almost everywhere you look, these contests could go in either direction. No clear pattern is emerging in many of these races, which means a fall campaign in which strategists in both parties alternately dream of emerging with control of the chamber or sweating that the outcome will leave them in the minority.

If control of the Senate comes down to a race-by-race contest, Republicans have a slight upper hand. To gain the majority, Democrats need to protect the five incumbents in the reddest states and win two of four competitive races where Republicans hold the seat. Each defeated incumbent makes ultimate victory significantly more difficult.

Right now, several Democratic incumbents are in jeopardy, starting with Sen. Heidi Heitkamp of North Dakota, who is being challenged by Republican Rep. Kevin Cramer. Heitkamp won her last race with just 50.5 percent of the vote. Republicans express more confidence about this contest than almost any other. The Cook Political Report lists it as a toss-up.

Not far behind in degree of difficulty for the Democrats are the races in Missouri and Indiana, where Democratic Sen. Joe Donnelly faces Republican businessman Mike Braun. Republicans like to recall they were given little chance initially of defeating former senator Evan Bayh in his 2016 comeback attempt, and they like their chances in this race. But some Republicans note that Donnelly is running a smart campaign.

Two other Democratic incumbents in toss-up races are Sen. Jon Tester of Montana and West Virginia’s Joe Manchin III. Trump has campaigned in their states in an effort to rally his voters to turn out in November. But of the five Democrats in the reddest states, these two appear, today, in marginally better shape than their colleagues in North Dakota, Missouri, Indiana and Florida.

Holding all five of those seats would still mean Democrats have to knock off two Republicans. Their prospects are brightest in Nevada, Tennessee and Arizona, where Republican Rep. Martha McSally is facing Democratic Rep. Kyrsten Sinema for the seat now held by retiring Republican Sen. Jeff Flake.

None of those states is particularly easy for the Democrats. Presidentially, Nevada is a truly purple state, trending blue perhaps because of changing demographics. Democrats like to think the same is happening in Arizona, but it hasn’t yet jelled. Tennessee, meanwhile, is moving in the other direction, redder and redder. It is not the same state as when Bredesen won the governorship.

Texas is the other state where Democrats have seen their prospects improve, thanks to a campaign by Democratic Rep. Beto O’Rourke that has captured national attention. O’Rourke eschews many of the conventions of modern politics, preferring his own instincts to that of consultants. He’s turned his challenge to Republican Sen. Ted Cruz into a competitive contest, albeit in a state that remains conservative and predominantly Republican.

So running the map state by state underscores the challenge to the Democrats. They need near-perfect campaigns to offset the GOP’s built-in advantages. Looking at things from that perspective, it’s no wonder that Republicans think, in the end, they will hold their majority in the Senate or even add a seat or more.

The question is whether there are larger forces at work that could turn things in the direction of the Democrats, things that have less to do with who runs the best television commercial or knocks on the most doors or has good debate performances.

“What keeps me up at night is the ‘overriding force’ possibility,” a Republican strategist emailed on Friday. “In 2014 and 2016, our party benefited from late-in-cycle movement that tipped nearly all the close races our way. You could see something like that developing this fall — not a wave, just a shift — that could flip close races in the Democrats’ direction.”

Today no one can say whether that will be the case. But the very existence of a series of races that are as close as they are right now, and the possibility that they stay that way over the coming weeks, suggests that the campaign for the Senate deserves plenty of attention.