Control of the U.S. Senate may come down to Nevada, where a slow ballot count entered its final act Saturday in the nail-biter contest between Democratic Sen. Catherine Cortez Masto and Republican challenger Adam Laxalt.

Saturday is the last day that mail ballots can arrive and be counted under the state's new voting law. Election officials were hustling to get through a backlog of tens of thousands of ballots to determine the race's winner, with the state's largest county saying it hoped to be effectively done by the evening.

The Nevada race took on added importance after Democratic Sen. Mark Kelly was declared the winner of his reelection campaign in Arizona on Friday night, giving his party 49 seats in the chamber. Republicans also have 49.

If Cortez Masto wins, Democrats would maintain their control of the Senate given Vice President Kamala Harris' tiebreaking vote. If Laxalt wins, the Georgia Senate runoff next month would determine which party has the single-vote Senate edge.

Cortez Masto was only a few hundred votes behind Laxalt, with most of the remaining uncounted ballots in heavily Democratic Clark County, which includes Las Vegas. Democrats were confident those ballots would vault their candidate into the lead.

Laxalt has said he expects to maintain his advantage and be declared the victor. But on Saturday he acknowledged in a tweet that the calculus has changed because Cortez Masto had performed better than Republicans expected in Clark County ballots counted over the past few days.

Read more: Democrats hold small but shrinking lead in key Arizona races

“This has narrowed our victory window,” he tweeted, acknowledging the race comes down to the final Clark County ballots.

“If they are GOP precincts or slightly DEM leaning then we can still win,” Laxalt tweeted. “If they continue to trend heavy DEM then she will overtake us.”

If a winner isn't clear by the end of the day on Saturday, attention would shift to a few thousand more ballots that could be added to the totals early next week. Mail ballots with clerical errors can be “cured” by voters until the end of the day Monday, and then added to the totals. And a few thousand provisional ballots also remain, votes that election officials must double-check are legally countable by Tuesday before they can be tallied.

“We know that this is a serious count. There are people nationwide who are looking to these results,” Joe Gloria, the registrar in Clark County, said at a press conference Saturday. “We know that people need to see that count. We're not going to delay it any further.”

Gloria said all 22,000-plus remaining ballots would be tabulated by Saturday evening. “They’re all being counted,” Gloria said. “My vaults are empty.”

Still, state law requires a relative handful of ballots to linger. In Clark County, there are also 7,100 ballots being “cured” and 5,555 provisional ballots. The county accounts for three-quarters of Nevada's population.

Gloria noted that it takes a couple of cycles to adjust ballot-counting to the all-mail system that Nevada switched to during the 2020 pandemic. He also noted that state law requires him to accept ballots until Saturday. “We couldn't be done any earlier, even if we wanted to,” Gloria said.

In another key race, Democratic Gov. Steve Sisolak lost his reelection bid to his Republican challenger, sheriff Joe Lombardo, on Friday night.

Nevada, a closely divided swing state, is one of the most racially diverse in the nation, a working class state whose residents have been especially hard hit by inflation and other economic turmoil.

Roughly three-fourths of Nevada voters said the country is headed in the wrong direction, and about 5 in 10 called the economy the most important issue facing the country, according to AP VoteCast, a survey of 2,100 of the state’s voters.

Read more: GOP moves closer to winning the House; the Senate's fate may depend on a runoff

Voters viewed the economy negatively, with VoteCast finding nearly 8 in 10 saying economic conditions are either not so good or poor. Only about 2 in 10 called the economy excellent or good. And about a third of voters said their families are falling behind financially.

But that didn’t necessarily translate into anger at President Joe Biden or his party. About half considered inflation the most important issue facing the U.S., but they were evenly split over whether they think higher prices are due to Biden’s policies or factors outside his control.

According to VoteCast, 7 in 10 voters in Nevada wanted abortion kept legal in all or most cases, and Cortez Masto and other Democrats made preserving the right a centerpiece of their campaigns.

Republicans, however, relentlessly hammered the economic argument, contending it was time for a leadership change. They also sought to capitalize on lingering frustrations about pandemic shutdowns that devastated Las Vegas’ tourist-centric economy in 2020.

On Thursday morning, The Associated Press declared Republican Stavros Anthony the winner in the lieutenant governor race, while Republican Andy Mathews was elected state controller.

The state’s lone Republican congressman, Mark Amodei, easily won reelection in his mostly rural district in northern Nevada. The state’s three Las Vegas-area Democratic members of the House were also reelected.