Alaskans will issue a final decision Tuesday on a controversial new oil tax regime, and pick a Republican challenger to Democratic incumbent U.S. Sen. Mark Begich.

Tuesday's decisions come after the most expensive primary campaign in Alaska history, with the referendum on a 2013 oil tax cut drawing some $15 million in spending, almost entirely from industry groups trying to keep the new tax system in place, and more than $15 million from the campaigns of the GOP Senate candidates, along with their allies and opponents.

Voters will also choose party nominees for state races and a dozen races for seats in the state Legislature.

Statewide, more than 400 polling places open at 7 a.m. on Tuesday, and close at 8 p.m.

The hotly contested referendum on the oil tax repeal -- the only proposition on Tuesday's ballot -- follows a 2013 vote by the state Legislature that instituted a new tax system for oil producers. The new law -- Senate Bill 21, or the More Alaska Production Act -- went into effect this year, replacing Alaska's Clear and Equitable Share, which was signed into law in 2007 by then-Gov. Sarah Palin.

Oil companies said the former law was discouraging investment, thanks to tax rates that rose as oil prices increased. Last year, Gov. Sean Parnell worked with legislators to pass the new tax regime, which cut taxes in a bid to get the oil companies to spend more and boost production.

Opponents of the tax cut, a loose coalition led in part by former Democratic state Sen. Vic Fischer, put the repeal question on Tuesday's ballot after gathering more than 50,000 petition signatures.

A yes vote will repeal Senate Bill 21 and reinstate ACES. A no vote will keep Senate Bill 21 in place.

On Monday afternoon, groups on both sides of the issue said they were campaigning aggressively down to the wire. Companies supporting the tax cut were shuttling workers to the polls in vans -- as recently suggested by one anti-repeal group -- but officials said they had not told employees how to vote.

In the space of one hour at an Anchorage early voting site, five different vans arrived, hauling workers from BP, Lynden, Cook Inlet Region, Inc., Saltchuk, and Trailercraft/Freightliner.

A couple of drivers said they'd already brought 30 people in multiple trips, including in Saltchuk's 16-seat van that came with chilled bottled water and sports drinks in a cooler.

Elsewhere, campaign officials on both sides of the vote said they'd stepped up phone calls, sign-waving and other outreach efforts.

Willis Lyford, campaign spokesperson for Vote No On 1, said he'd spent the weekend in Wasilla and South Anchorage, knocking on doors.

Nick Moe, volunteer coordinator for Vote Yes! Repeal the Giveaway said the group had called more than 4,000 voters in the last two days. After running out of independent voters to call, they were reaching out to Republican women, Moe said.

"We'll be working until late into the evening," Moe said.

He added the decision to reach out to that demographic came after a pair of prominent women voiced their support for a yes vote: Palin and Kathleen Miller, the wife of Republican U.S. Senate candidate Joe Miller.

(Miller has said he will vote no on the referendum, and spokesman Randy DeSoto said Monday that Miller's position had not changed.)

The Senate race

The U.S. Senate candidates -- Miller, former state attorney general and natural resources commissioner Dan Sullivan, and Lt. Gov. Mead Treadwell -- spent Monday at events around the state, with Treadwell unveiling a pair of last-minute endorsements, including one from astronaut Buzz Aldrin.

Miller paid a visit to a senior center in Wasilla, while Sullivan and his family made their last stops on a 750-mile RV tour, which at one point Monday afternoon was held up by a crash between a semi truck and a tour bus that stopped traffic on the Parks Highway.

The Senate campaign has taken place on two different stages this year.

The mass media battle, waged primarily through television ads, has largely pitted Begich, the incumbent, against Dan Sullivan, the GOP candidate who's attracted the most money and support from establishment Republican groups.

Begich and his allies have relentlessly attacked Sullivan, who has responded with his own attack ads and reminders that the attention from Democrats suggests he's the most viable Republican nominee.

The two sides have spent more than $15 million on their efforts, according to data from the Federal Election Commission and the nonpartisan Center for Responsive Politics -- though those figures do not include spending by so-called "dark money groups" that have spent millions more, according to Begich's campaign.

The other battle has taken place on the radio, on social media, and at forums and meetings with voters, where the three Republicans have outlined their policies, drawn distinctions between one another and increasingly lobbed attacks at each other.

Recent polls have shown a tightening in the early lead Sullivan had over his two main Republican challengers, Lt. Gov. Mead Treadwell and Joe Miller.

Heavy early voting

Officials said Monday that early and absentee voting were running ahead of the pace set in 2010 -- the last time there was a primary election in a non-presidential year.

More than 8,100 people had voted early as of 3 p.m. Monday, compared to 5,100 for all early voting in 2010, according to Gail Fenumiai, the director of Alaska's elections division.

"That's going to continue to rise," she said.

The two sides of the oil tax debate, amplified by a flood of mailers and advertising, have portrayed Tuesday's vote as one of the most important since statehood, with the fate of Alaska's economy in the balance.

"Parnell will give your PFD to ConocoPhillips, Exxon, and BP," reads one mailer sent recently by Vote Yes!, referring to the annual checks issued to Alaskans out of an investment fund established after the original discovery of oil on the North Slope in the 1970s.

But Gunnar Knapp, director of the University of Alaska Anchorage's Institute of Social and Economic Research, said that Tuesday's vote merely concludes one phase in the state's evolving debate over oil taxes.

"If in retrospect it looks like we've made a stupid choice, then certainly, all of history says that we'll look to modify that choice," he said. "The chances are the people and the Legislature will see it soon enough, and begin to adjust."

There's ample uncertainty about the potential impact of both tax structures, Knapp said, and the outcomes may not end up being very different between the two. Both regimes face a "major challenge" in turning around the state's longstanding oil production decline, and generating enough revenue to close Alaska's budget gap, predicted to be $1 billion or more over each of the next several years.