File, Associated Press
This combo of file photos shows South Dakota U.S. Senate candidates, from left: Republican Mike Rounds; independents Larry Pressler and Gordon Howie and Democrat Rick Weiland. They are running for the seat being vacated by retiring Democratic Sen. Tim Johnson.

PIERRE, S.D. — With the nation watching the tight race for South Dakota's open U.S. Senate seat, the four candidates are girding for the final stretch before Nov. 4.

Republican Mike Rounds, Democrat Rick Weiland and independents Larry Pressler and Gordon Howie are vying to replace retiring Democrat Tim Johnson.

Republicans see South Dakota as a must-have in their push to net six seats and take control of the chamber.

Many figured Rounds would win handily, and until recently the race hadn't been considered competitive. But in the second week of October, the campaign arms of both parties invested $1 million each into TV advertising.

Republicans have maintained that Rounds has clung to a comfortable lead, even though it isn't entirely clear just how close the race is; there's been little public polling. Meanwhile, Pressler continues to run an outsized campaign on a shoestring budget.

Rounds' opponents say his support has eroded amid criticism of the EB-5 visa program, which operated while he was governor and allowed wealthy foreigners to invest in South Dakota projects that created jobs in exchange for U.S visas.

"The people I'm talking to in South Dakota, frankly, governor, just don't trust you," Howie said during Thursday night's debate at the University of South Dakota in Vermillion. But Rounds has continuously defended the effectiveness of the program, arguing it created thousands of jobs in South Dakota.

The campaigns for Rounds, Pressler and Weiland said the candidates would continue appealing to voters in the days before Nov 4.

The Rounds' camp plans nonstop appearances across the state.

Meanwhile, Weiland's team said he would continue a campaign that's in the middle of its second tour of every town in South Dakota. And Pressler appealed to voters during Thursday's debate to print the lawn signs his campaign can't afford to, and also noted he's relying on a grass-roots groundswell unprecedented in South Dakota politics.

As of Oct. 15, Rounds' had the largest campaign-cash stockpile — about $667,000, Weiland sat on $334,000 and Pressler had about $169,000 in the bank.

The candidates have clashed over immigration reform, partisan gridlock in Washington and policies to keep Social Security solvent, as well as taxes and health care reform. Weiland has cast himself as a crusader against money in politics, while Rounds has tied his opponents to President Barack Obama and government bloat.

Pressler, who has also railed against government corruption, has positioned himself as the candidate who can reach across the aisle.

But what is unclear in the short few days before the election is whether Weiland can keep up his momentum in this Republican-dominated state against Rounds and from which party the majority of Pressler's support will come.

"I think we made it into a real Senate race, which was going to be a coronation more or less," Pressler said.