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 Donald 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 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.

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 re-election 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. 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 tossup.

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.

So Democrats 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.

“What keeps me up at night is the ‘overriding force’ possibility,” a Republican strategist e-mailed 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.