The math for Senate control is pretty simple. For Democrats, it isn’t adding up.

The Democrats have a 51-49 majority in the Senate, but Republicans are expected to flip West Virginia — where Joe Manchin is retiring. Any additional Republican victory in November in seven red or purple states currently held by Democrats would ensure G.O.P. control of the chamber, provided the Republicans hold their own states.

This morning’s New York Times/Siena College poll finds Republicans poised to flip that additional seat: Montana

Tim Sheehy, a Republican, leads the longtime Democratic incumbent, Jon Tester, by seven percentage points, 52 percent to 44 percent (figures rounded).

Before today, Democrats led nearly every Times/Siena poll of the contested Senate races for months, including in Ohio — where Sherrod Brown led by four points in a state Donald J. Trump won by eight four years ago. A positive Montana result for Democrats would have kept that string going, giving them a path to Senate control. They didn’t get it.

Even so, the Montana result isn’t a surprise. Mr. Tester hasn’t led in a public poll since mid-August, and most of the data hasn’t shown an especially close race. His path to victory — and therefore the Democratic path to Senate control — looked daunting even before he fell behind in the polls, as Mr. Trump won the state by 16 points in 2020. The poll finds Mr. Trump ahead of Kamala Harris by a similar 17 points today.

Without Montana, the Democratic path to the Senate would require flipping a red state. The Times/Siena polls suggest it won’t be easy. On paper, the two easiest options ought to be Texas and Florida, but the polls find the two Republican incumbents ahead — Ted Cruz by four points in Texas, and Rick Scott by nine in Florida.