The race, statistically a dead heat, is one of the handful of key Senate contests that could determine whether the Republicans achieve the unexpected -- regain control of the Senate, where the Democrats hold just a one-seat majority.
"Laura and I are here because the people of this important state have some important decisions to make," Bush said.
"We believe it is in the best interests of South Dakota and the best interests of America to elect John Thune to the U.S. Senate.
"I'm also here because I'm seeking some allies (in Washington), people I can count on ..."
Thune, a three-time congressman, is running neck-and-neck against Democratic incumbent Tim Johnson. A recent Mason-Dixon poll taken for South Dakota media clients Oct. 25-27 showed Thune just 2 points behind Johnson, 45 percent to 47 percent, respectively. The error of margin for the poll of 805 likely voters was plus or minus 3.5 percent.
Sunday's boisterous rally in the Sioux Falls Convention Center marked the second visit by Bush to the state in support of Thune and other GOP candidates in less than a week. On Thursday he held a rally in Aberdeen, hometown of Daschle.
On Sunday afternoon, Laura Bush, barnstorming on her own, accompanied Thune and his wife around the city, meeting voters and underlining the president's support of the candidate.
Bush has repeatedly complained in rallies how his agenda for America has foundered in the Democratically controlled Senate. Confirmation of federal judicial nominees is stalled, he said, affecting the quality of justice; a plan for prescription medication for seniors is stalled, creation of a Department of Homeland Security is mired in an argument over the union rights of some workers, and making tax cuts permanent is going nowhere, he says.
Each issue has its place in a template, stump speech that only varies in the names of the local candidates Bush plugs and the emphasis he gives any particular subject.
Bush is not running in the election, but his presence in the contest is unmistakable.
Earlier Sunday, Bush spun his remarks in St. Paul, Minn., where another crucial Senate contest is taking place. The prize is the Senate seat of Democrat Paul Wellstone, killed in a plane crash last month.
Latest public opinion polls show the Republican candidate, former St. Paul Mayor Norm Coleman, in what amounts to a statistical dead heat against the man chosen by Democrats last week to take Wellstone's place on the ballot -- former Senator and former Vice President Walter Mondale, who came out of an 18-year retirement to carry the Democratic-Farmer-Labor Party banner.
Coleman, since Mondale's entry into the race, is campaigning hard as a candidate with a modern and forward-looking agenda, not a politician from the past.
Any initial dip Coleman suffered following the death of Wellstone has apparently dissipated, according to a poll for the local media.
The St. Paul Pioneer Press and Minnesota Public Television poll statewide of 625 likely voters on Oct. 30-Nov. 1 showed Coleman leading Mondale by 6 points -- 47 percent to 41 percent, with 10 percent of voters undecided. With a plus or minus margin of error of 4 percent, the poll literally puts the two neck-and-neck.
A poll by the Minneapolis Star Tribune of 600 likely voters, taken last Wednesday through Friday, shows the numbers reversed -- 46 percent for Mondale, 41 percent for Coleman. The margin of error was plus or minus 3.2 percent, again making the race simply too close to call.
Bush, speaking at the Excel Arena in St. Paul, modified his remarks to stress Coleman's leadership and his vision for the state and the country. Coleman, he said, was a leader of the future, not of the past -- a clear swipe at Mondale.
A senior administration official, speaking to reporters on Air Force One on Sunday, said the outcome of Nov. 5 -- whether the GOP regains control of the Senate and keeps a narrow majority in the House -- is going to be a tight-run affair.
"... This is going to be settled by a relatively small number of votes, potentially, and a relatively small number of contests in both the House and the Senate. And it's going to be a close election," he said.
In midterm elections in a president's first term, the historic precedent is for his party to suffer reversals at the hands of the public.
In this election, the GOP may defy precedent by keeping their razor-slim majority in the House, where all 435 seats are up for grabs. Gaining control of the Senate would be a victory the president could clearly relish as he looks ahead for the next two years.
Besides the 34 Senate seats up for grabs, 36 governorships are in contest.
Traveling to Minnesota from Tampa, where he shared the stage Saturday night with his brother Jeb, running for re-election as governor, the president stopped in Springfield, Ill., home of Abraham Lincoln.
In a stump speech he bannered his administration's agenda, the attributes of the Republican candidates in the state, and his need for political allies in Washington.
The rally was held in the Springfield Police Armory, a block-long building on Madison and Second streets.
While the president rocked the crowds with his now-familiar stump remarks, a small group of between 60-70 people stood on a nearby street corner, holding signs protesting a possible war with Iraq.
"No War," one said. "Mr. President, Create Peace Now," said another.
"I'm here because I'm totally opposed to a war against Iraq," Susan Faupel, a 46-year-old social worker, told United Press International.
"I'm totally against using terror to stop terror, and that's what we are going to do." Iraq is a regular feature in Bush's addresses, in which he repeats the threat Iraq and its suspected weapons of mass destruction pose, the challenge he issued to the United Nations, and his resolve to disarm Saddam Hussein if the United Nations fails to "get the backbone" to act to enforce its disarmament resolutions.
At St. Paul's Excel Center, home of the city's professional hockey team, as many as 10,000 gathered to hear the president and see a tangible sign of his support for Coleman.
The demonstrators were there also. About 100-150, chanting "give peace a chance" and "we don't want your corporate war" on a nearby street corner, ignored by the Republican faithful.
There were no ant-war or anti-Bush demonstrators in Sioux Falls.
The Bushes were later flying to Cedar Rapids, Iowa, where they would begin Monday's campaign tour with a rally before heading to Missouri, Arkansas and Texas.
Monday will be Laura Bush's 56th birthday, a point Bush made in the Sioux Falls rally.
The crowd cheered and broke into a spontaneous rendition of "Happy Birthday."