PIERRE, S.D. (AP) — When U.S. Sen. Tim Johnson announced he wouldn't seek re-election — putting a seat in conservative-leaning South Dakota in play — national Democrats had a few candidates in mind to replace him.

None of them was the eventual nominee, and one of the first people to tell you that would be Rick Weiland, who's locked in an unexpectedly competitive race.

A businessman and former aide to ex-Sen. Tom Daschle, Weiland was ignored by national Democrats and Republicans alike until recently. In the vacuum, he's run an unabashedly liberal campaign: He has touted the benefits of President Barack Obama's health care overhaul and run ads in which he plays guitar and sings folksy lyrics decrying income inequality and the influence of big money in politics.

"Some of us believe corporations and billionaires that already have more, deserve more ... The rest of us believe it's time we got a break instead," Weiland, 56, said in his most recent TV ad.

His populist approach hasn't changed in the final few weeks, but the state of the four-way race has.

Initially expected to be won at a canter by former Gov. Mike Rounds, the Republican has struggled. Independent candidate Larry Pressler, a former Republican congressman, has found traction. A fourth candidate, tea party-inspired independent Gordon Howie, also is in the mix.

The GOP is counting on South Dakota as part of a push for the majority; it needs six seats to retake the Senate.

But Rounds has been accused of mismanaging a state-run federal visa program that operated while he was governor, through which wealthy foreigners could invest in projects in rural South Dakota that produced jobs in exchange for U.S. visas.

Weiland and Pressler have benefited from Rounds' wobbles, and the national campaign arms of Senate Democrats and Republicans have taken notice — investing $1 million apiece. Outside groups, including Mayday PAC, which supports campaign finance restrictions, also have invested heavily in Weiland.

Weiland's campaign received another symbolic boost this week: It outraised Rounds in the most recent fundraising period. Weiland's finance report, released Friday, showed he took in about $330,000 between Oct. 1 and Oct. 15, compared to Rounds' roughly $269,000. Rounds has more cash on hand, though, with $667,000 compared to Weiland's $334,000.

Another sign of the race's tightening has been on the airwaves, where Rounds, who had never run a negative ad in either of his gubernatorial campaigns, targeted Weiland and Pressler.

For months, Weiland has toiled quietly, boasting that he's been to each of South Dakota's incorporated cities twice and making reduced campaign spending a top issue.

With fresh focus on the race, Weiland has doubled down on his message. At a debate this week in the far southeastern South Dakota city of Vermillion, he repeated his support for comprehensive immigration reform and floated the prospect of additional taxes to fully fund Social Security. At another point, he said plainly he would not vote for Senate Majority Leader Harry Reid — but has no plans to vote for Republicans, either.

"Frankly, both political parties are just too beholden to big money," Weiland said at the event, hosted by the University of South Dakota.

Rounds countered that Weiland would be a "vote for Obama," calling Weiland's views too liberal for South Dakota.

The Democrat's path to victory is complicated, and he'll hope to win with about a third of the vote, with Pressler and potentially Howie pulling enough votes from Rounds. Weiland's supporters tend to be a minority within South Dakota's mostly conservative voter base, and many back him because they believe he will be a reliable Democratic vote.

Sioux Falls resident Rayna Fritz, 55, said Weiland's populist views put him in line with other liberal senators she supports, such as Elizabeth Warren, D-Mass. and Bernie Sanders of Vermont, an independent who caucuses with Democrats.

Weiland's idealistic approach inspires other voters, such as Patricia Lawlor, who'd been disillusioned with politics for several years before hearing Weiland's anti-big money pitch on the stump.

"All of a sudden I got excited, and I was like, 'I haven't felt this excited about politics in so long,'" the 51-year-old Rapid City resident said.

Others who might be inclined to support Weiland said they were concerned he couldn't win, and are considering a pragmatic vote for Pressler. Jil Jennewein, 56, of Rapid City, said she has been a loyal Democrat but backs the Republican-turned-independent because Weiland has let outside groups advertise for him.

Plus, Pressler might be more electable, she said, adding. "Republicans (here) don't vote Democrat ever."

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Jackson reported from Washington, D.C.