Norimitsu Onishi / New York Times News Service

RENO, Nev. — Rep. Shelley Berkley, battling for a Senate seat, flew to northern Nevada hours after appearing with Vice President Joe. Biden at a rally in her Las Vegas district. On Oct. 19, before returning south for the start of early voting, Berkley crisscrossed Washoe County, the battleground where her image has been largely shaped by a barrage of television advertisements from her rival attacking her ethics.

With Berkley, a Democrat, locked in a very tight race for one of the most fiercely fought Senate races in the country, her Republican opponent, Sen. Dean Heller, was calculating that he could scrape his way to re-election with an unrelenting focus on the ethics cloud surrounding her, analysts said.

Heller made no public appearances during the week, but his ubiquitous television ads have hammered at the congresswoman, who is being investigated by the House over allegations that she used her office to help her husband’s medical practice.

The race has remained tight despite the investigation, experts said, because of Berkley’s popularity in her district, which she has represented for seven terms, her tenacious campaigning and the formidable support of Sen. Harry Reid’s muscular machine. In the early morning cold, Berkley sought votes from Teamster members at a United Parcel Service center before heading to a breakfast here with a receptive group of older residents.

“I will stand with you, I will work with you, and I will fight for you,” she said, pledging to protect Medicare and Social Security.

Wider implications

A victory by Heller is critical to Republican hopes of wresting the Senate from Democratic control and dislodging Reid, this state’s senior senator, as majority leader. A former congressman who was elected to a safely Republican district here in 2007, Heller was appointed last year to succeed John Ensign, another Republican, who resigned from the Senate over a sex scandal.

Still reeling from the nation’s highest unemployment rate and one of its highest home foreclosure rates, Nevada has emerged as one of the most contested states in the presidential battle, and the Senate race has also become a proxy, pitting two candidates with records of voting along partisan lines.

A onetime stockbroker who referred to the unemployed as “hobos” in an unguarded moment, Heller, 52, is a conservative who has attacked Berkley for supporting President Barack Obama’s economic policies.

But more than anything, Heller’s television ads, as well as those financed by super PACs pouring money into Nevada, have aimed at Berkley’s ethics problems. A House panel is investigating whether Berkley wrongly intervened with Medicare officials in 2008 to keep open a troubled kidney transplant center with ties to her husband’s practice.

The investigation has hurt Berkley among voters like Steven Foremaster, 47, a lawyer who was watching the Hispanic parade. An Obama supporter, he said Berkley would not get his vote because of the investigation.

“That was the main thing for me,” he said.

In an interview, Berkley repeated her position that Nevadans worked together to save the state’s only kidney transplant center and that her husband’s ties to it were widely known.

Speaking of her opponent’s focus on her ethics — his latest ad called her “one of the most corrupt members of Congress” — she said, “He doesn’t have anything else.”

But Heller clearly believes it is enough to win, analysts said.