The Republicans have a great opportunity to take back the Senate, even without an anti-Democratic wave. This year’s Senate contests are being fought on Republican-leaning turf. There are seven Democratic-held Senate seats in states won by Mitt Romney, more than the six needed to retake the chamber. There are also a handful of competitive races in presidential battlegrounds. These are contests the Republicans could win under neutral or even Democratic-leaning conditions.

But the Republican task will become much more difficult if there isn’t a G.O.P. wave. The distinguishing feature of this year’s Senate battleground is a broad and competitive playing field where, so far, the Republicans haven’t broken through. They haven’t yet locked down seats like Arkansas or Louisiana, where Democratic incumbents remain doggedly competitive in places where Mr. Romney won by around 20 points in 2012.

If there isn’t a Republican wave, this year’s Senate contest will devolve into the electoral version of trench warfare. In that case, Democrats could certainly hold the line. There’s a long record of Democratic incumbents outperforming Democratic presidential candidates in the South, including as recently as 2012. It remains to be seen whether the Republicans can win a competitive federal contest under neutral conditions in states like Colorado, Iowa and Michigan.

This dynamic — where the Republicans have enough opportunities to make them the overall favorites, even as they struggle to break through — is captured well by Leo, the Upshot’s Senate prediction model. It gives Republicans a 60 percent chance to take over the Senate, but that’s mainly because the Republicans have so many opportunities to find the six states they need. Leo makes Republicans a favorite in only five states, one short of a majority. An anti-Democratic wave would make this election easy for the Republicans. It will be a very long election night without one.