Both candidates made last-minute pitches Tuesday, with Amodei appearing at a GOP women's luncheon and Marshall making a last-minute appeal to supporters in an email.
"If Democrats vote, Democrats win," she wrote.
The House district has long been regarded as the GOP's lone safety post in Nevada. The state's other congressional districts are urban and largely Democratic.
The special election was brief but heated. Marshall slammed Amodei for supporting tax increases as a state lawmaker and sought to portray him as a foe of Medicare in a series of TV attacks. Amodei, meanwhile, often linked Marshall to Obama and other Washington Democrats.
The race was Nevada's first House special election race, and the initial uncertainty surrounding the rules of the race gave Democrats hope that they could make history by taking the traditionally GOP seat.
The state's chief election officer originally ruled that the race would be a free-for-all, and at one point, more than 30 candidates were expected to enter the open contest. The Nevada Supreme Court ultimately said that major political parties should choose their candidate.
Also on the ballot were Tim Fasano, an Independent American Party candidate, and Helmuth Lehmann, an independent businessman who gathered signatures to get on the ballot.
The contest attracted political heavyweights early on, with former President Bill Clinton and Reid pushing for Marshall as House Speaker John Boehner and Nevada's Republican Gov. Brian Sandoval rallied for Amodei.
But national Democrats seemed to eventually give up on the campaign, directing zero dollars toward lifting Marshall, even as outside Republicans spent more than $1 million on TV attacks that tied Marshall to Obama.
Robby Mook, executive director of the Democratic Congressional Campaign Committee, downplayed the lack of financial support Tuesday afternoon as the polls were about to close.
"We think she's done an outstanding job," he said. "She's really run a flawless campaign."
Amodei will serve the remainder of Heller's term and will have to seek re-election in 2012 to keep the seat.
Associated Press writer Scott Sonner in Reno contributed to this report. Sonner and Chereb reported from Reno.
- Nepal quake: Nearly 1,400 dead, history...
- Photos: Celebrating 25 years of spectacular...
- Former Romney finance chairman courting a...
- Utah construction companies fined, ordered to...
- Utah in top 10 for visitor spending at...
- 10 dead as quake and avalanche sweep Mount...
- Powerful earthquake strikes Nepal: Things to...
- Chilean village a ghost town after Calbuco's...
- Former Romney finance chairman courting... 49
- Same-sex marriage opponents urge... 37
- House Benghazi panel calls Clinton to... 29
- Gay marriage a difficult issue for GOP... 28
- Obama sounds alarm on climate damage in... 25
- Yellowstone supervolcano is super big,... 24
- American, Italian hostages killed in US... 22
- Hatch, Lee split on new attorney general 12