ANCHORAGE — Leading into Tuesday’s primary, Alaska’s gubernatorial campaigns have been largely overshadowed by hard-driving runs for a U.S. Senate seat and a ballot referendum to repeal the current version of the state’s oil tax.

Expect the race for governor to fire up for the general election.

Republican incumbent Gov. Sean Parnell and Democratic challenger Byron Mallott are not expected to face major challenges in their respective primaries, and have waged low-key campaigns. Each is expected to advance to the general election, setting up a three-way race when they are joined on the November ballot by independent candidate Bill Walker, who finished second behind Parnell in the 2010 Republican gubernatorial race. Libertarian Carolyn Clift is also running.

Walker, who is bypassing the primary, opted to gather signatures to qualify as an unaffiliated candidate rather than take another run at the GOP nomination.

Parnell campaign spokesman Luke Miller said the governor is taking nothing for granted and his campaign will escalate in the coming weeks. A week before the primary, Parnell, 51, reported $157,000 in media buys, of which nearly $59,000 is partial payment for television ads after the primary.

“We’re running a tough campaign,” Miller said. “Listen, we’ve got not just one challenger, but we’ve got two very credible challengers.”

Political observers say Parnell holds an edge, and not just as the incumbent, but because any anti-Parnell vote will be fractured between his competitors. Of the three candidates, Parnell brought in the most money in the reporting period between July 19 and Aug. 9, raising more than $35,000 to close with $300,000 on hand. Mallott raised more than $23,000, closing with more than $47,000. Walker raised more than $18,000, ending the period with nearly $99,000.

Mallott’s campaign and Walker disagree, saying their campaigns are picking up momentum. Mallott, 71, is a former chief of the Alaska Permanent Fund Corp. and a former Yakutat mayor. Walker, 63, is an attorney focused on oil and gas issues with non-industry clients, as well as a former mayor of Valdez, the terminus for the 800-mile trans-Alaska oil pipeline.

Laury Roberts Scandling, a spokeswoman for Mallott, said the campaign is expected to ramp up after the primary clears the question of who will be the GOP Senate candidate in the general and whether voters will choose to repeal a system championed by Parnell that lowered taxes for oil companies operating in Alaska. As for splitting the vote, Parnell and Walker are more closely aligned, Scandling said.

“Our take has always been you have two Republicans,” she said. “The fact is Byron is the only moderate candidate when it comes to social issues that have generally widespread acceptance by Alaskans.”

Walker, who has long pushed for an “all-Alaska” major natural gas pipeline, has said he would aggressively lead the fight to develop Alaska’s resources. In announcing his candidacy last year, he said the Legislature’s passage of Senate bill 21, which created the new oil tax system, played a role in his decision to seek office since the oil companies aren’t required under the bill to increase production.

A week before the primary, Walker said passage of the oil tax referendum would be a vote of no-confidence in Parnell, a former state government relations director for ConocoPhillips in Alaska. Walker supports repeal and said he would offer leadership that are now lacking. However, he said he is more than a one-issue candidate, listing education, Medicaid expansion, domestic violence and fish management as other priorities.

“But when we are a state that generates 90 percent our revenue from oil and gas, from oil, it’s time we have someone with my expertise on our side of the table,” he said.

Of the three candidates, Parnell is the only one in favor of keeping the current oil tax system. If the repeal effort fails, it would be an indication that his challengers couldn’t use that issue against him, said Marc Hellenthal, a pollster in Anchorage not working for any of the candidates.

“If it passes, that would be an embarrassment, to put it mildly, and it would cost him some votes,” Hellenthal said. “How many votes is impossible to tell, until you poll afterwards.”