PORTLAND, Maine (AP) — Republican Charlie Summers and Democrat Cynthia Dill won their crowded primary elections Tuesday in the race for Republican Sen. Olympia Snowe’s soon-to-be-empty seat, setting the stage for a three-way race with a popular former governor that could shift the balance of power in the U.S. Senate.

Summers was leading with 30 percent of the vote followed by Bruce Poliquin with 22 percent, with 77 percent of the state’s precincts reporting in unofficial returns in the GOP primary. In the Democratic primary, Dill had 45 percent to 35 percent for Matthew Dunlap.

Dill said the Democratic Party is ready for somebody who will stand up and be a strong voice in Washington for the middle class and small businesses. She also had a message for King: “Bring it on.”

“Democrats are ready to have a torchbearer who states very clearly and articulately what the Democratic core values are and not try to fudge and play to everyone in the political spectrum,” she said.

The Republican and Democratic nominees will face Angus King, a former two-term governor, and several other independents in an important race that could shift the balance of power in the closely divided U.S. Senate.

Snowe took the political establishment by surprise when she announced Feb. 28 that she wouldn’t seek a fourth term, citing partisan polarization and political gridlock in the Senate.

All told, 10 Republicans and Democrats were vying against each other in primaries Tuesday.

Scott D’Amboise, who had intended to challenge Snowe in a primary, was soon joined by five other Republicans aiming for the spot. Big-name Democrats like U.S. Reps. Chellie Pingree and Mike Michaud and former Gov. John Baldacci considered joining the race but opted out, leaving four Democrats.

Joining D’Amboise, a small businessman from Lisbon Falls, on the Republican ballot were Summers, the secretary of state from Scarborough; Poliquin, state treasurer from Georgetown; and Bennett, a former Senate president and CEO of a corporate watchdog firm from Oxford, along with Attorney General William Schneider of Durham and Debra Plowman of Hampden, state senator and co-owner of a garage door business.

Democrats on the ballot were Dill, a state senator from Cape Elizabeth, and Dunlap, former secretary of state from Old Town, along with Rep. Jon Hinck of Portland and Ben Pollard, a Portland homebuilder.

King, who wasn’t even on Tuesday’s primary ballot, cast a long shadow on the campaign.

“For any of these candidates to knock him off is going to take a monumental effort,” said Mark Brewer, political science professor at the University of Maine.

Republicans were all but assured of holding onto the seat before Snowe’s departure. As an independent, King insists he can bridge the gulf between the parties. Senate Democrats currently hold a 51-47 majority with two independents who caucus with them.

In the primary campaign, Republicans focused much of their attention on King, a former Democrat, attacking him as a free-spending liberal and thinly disguised Democrat-in-independent’s clothing.

Democrats battled to show they that can produce a candidate strong enough to take on the Republican nominee as well as King, a self-made millionaire. Also on the ballot will be three other independents, including businessman Steve Woods of Yarmouth and tea party activist Andrew Ian Dodge.

On Tuesday, many voters said they viewed King as difficult to beat. And Democrats worried that the former member of their party could siphon off Democratic votes and give Republicans an advantage.

“The reality is that I do think Angus King will win the general election,” said Elizabeth Simpson, a Democrat who voted for Dill Tuesday and said she’d vote for King if necessary in November to keep the Senate out of GOP hands.

Peter Gaulke, a left-leaning Republican, said he couldn’t vote for any of the GOP candidates on the primary ballot so he wrote in King’s name. “I’d just like to see people be more pragmatic. Ideology has gone too far,” he said.

For the most part, campaign spending was relatively low, with only five of the candidates raising more than $100,000 and only a few buying television advertising. D’Amboise raised nearly $700,000, much from out-of-state donors; Poliquin raised more than $250,000, including $124,000 of his own money and in-kind donations.

While King is considered the likely front-runner in November, the mainstream parties can’t be discounted.

“One of the things about elections is no matter how circumstances look leading up to Election Day, anything can and sometimes does happen, until we get to the actual polls,” Brewer said. “Anybody who gets a major party nomination — any of these candidates has a shot.”