In sprawling Washoe County, the area stretching from Lake Tahoe to the Oregon border that is widely considered the most politically competitive in Nevada, a surge of enthusiasm in recent weeks has given Democrats hope that Senator Jacky Rosen, one of the party’s most vulnerable incumbents, can pull off a re-election victory.

“The energy is incredible right now,” Carissa Snedeker, the chairwoman of the Washoe County Democrats, said in a recent interview.

Farther south, in rural Douglas County, nestled in the shadow of the Sierra Nevada in the state’s southwestern corner, even Republicans and independents have come to the local Democratic office looking for volunteer opportunities and information on how to donate to Ms. Rosen.

“As long as I’ve been involved with politics here in Douglas County, this is the most enthusiastic, the most excited people have been,” Lori McKimmey, the chairwoman of the Douglas County Democratic Party, said before a recent meet-and-greet with the senator. “That being said, though, it’s going to be very close.”

Ms. Rosen, a first-term senator in a hotly contested state, is skipping her party’s convention in Chicago this week to focus on her race against her Republican challenger, Sam Brown, a former Army captain and staunch conservative, which is expected to be one of the most competitive and costly in the nation. But even as she keeps her distance from the top of the ticket, her allies say she has benefited from renewed enthusiasm among Democrats after Vice President Kamala Harris replaced President Biden as the party’s presidential nominee.

Ms. Rosen was on hand this month for Ms. Harris’s raucous rally in Las Vegas — the last in a weeklong series of swing state stops after the vice president announced Gov. Tim Walz of Minnesota as her running mate. The rally turned out to be one of the largest political events in the state’s modern history, drawing a crowd of more than 12,000 supporters, according to officials with the Harris campaign.