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Argus Leader, Jay Pickthorn, Associated Press
Former Republican presidential nominee Mitt Romney campaigns for U.S. Senate candidate Mike Rounds during a rally at the Best Western Ramkota exhibit hall in Sioux Falls, S.D. on Thursday, Oct. 30, 2014.

SIOUX FALLS, S.D. — Former Republican presidential nominee Mitt Romney stumped for U.S. Senate candidate Mike Rounds on Thursday as part of a national tour to energize the GOP base ahead of Election Day.

Rounds is up against Democrat Rick Weiland and independents Larry Pressler and Gordon Howie in a race that, against the odds, briefly turned competitive in this Republican-dominated state and has only recently drifted back into Rounds' favor. Romney is one of a few high-profile party leaders that Rounds has brought in.

South Dakota is viewed as a must-have in the Republican quest to net six seats and oust Democrats from control of the Senate.

"This is a chance where our strength as a small state is magnified significantly," Rounds said. "Today we have the opportunity to change the course of America."

The early front-runner in the campaign to replace outgoing Democratic Sen. Tim Johnson, the former governor's fortunes — and polling — had slipped in recent weeks. His opponents said it was due to 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.

But a poll from earlier this week showed he had a nine-point lead over Weiland.

Rounds appears to have heeded his shifting position, telling about 50 volunteers in a Sioux Falls campaign office Thursday: Don't stop working to get Republicans to the polls until the race is over — and won.

At the rally, Romney criticized President Barack Obama for slowing progress on the Keystone XL pipeline, which would cut through South Dakota in transporting oil from the Canadian tar sands to Nebraska. He also went after the president for the health care reform law, immigration policies and the administration's "hesitancy and weakness abroad."

Sen. John Thune, who has also been appearing with Rounds, told the crowd a Republican majority in the Senate is necessary to revive the chamber from being "a graveyard of good ideas."

Romney, who said he worked with Rounds when he was governor of Massachusetts, also praised the Republican for improving South Dakota's education system and boosting the state's economic climate.

Rally attendee Patti Giebink said she doesn't always agree with Rounds, but she believes he's honest. The 64-year-old Chamberlain resident said she's confident Rounds wouldn't get caught up in Washington politics and forget his roots.

At a restaurant in downtown Sioux Falls, the three Republican politicians circulated among tables of people. A couple who had just moved back to South Dakota asked Romney about his presidential ambitions in 2016.

"I'm trying to get some other people to run," Romney replied with a laugh. "I'm trying to get this guy elected. That's what I'm here for."