Former South Dakota Rep. John Thune (R) will announce a challenge to Sen. Tom Daschle (D) later today, according to a number of state and national Republican sources.

In anticipation of a Thune announcement, the Daschle campaign launched a new 60-second ad this morning and plans to stay on the air continuously through Election Day.

“We have spent a year building an organization that is strong and ready to compete with any opponent who runs,” said Daschle campaign manager Steve Hildebrand. “We couldn’t be better positioned.”

Thune’s decision to run ends more than a year of speculation about his political future.

Despite significant pressure to run in the special election to replace resigning Rep. Bill Janklow (R) — scheduled for June 1 — or to move up his Senate decision, Thune has kept to a timeline he set for himself just after his narrow 524-vote loss to Sen. Tim Johnson (D) in 2002.

Thune’s decision to run against the Minority Leader signals a go-for-broke effort by the South Dakota Republican to knock off one of the most high-profile figures in the Democratic Party.

He faces a daunting task as Daschle is expected to show more than $4 million in the bank in his year-end report and the most recent independent poll — conducted in early November — showed Daschle with a 6-point edge.

Hildebrand noted that in order for Thune to raise $10 million — the amount Daschle expects to bring in — he must raise $32,000 every day between today and Nov. 2. (In his 2002 race against Johnson, Thune raised and spent nearly $6 million.)

Republican strategists close to Thune express little concern about his ability to raise enough money to be competitive with Daschle since the Minority Leader is broadly disliked by Republican activists nationally, many of whom will be willing to give in order to see him defeated.

And, they contend, private polling continues to show the race neck and neck despite several months of positive ads run by Daschle.

The latest of these ads hits the airwaves today, touting Daschle’s work on behalf of a South Dakota woman with a heart condition who was in danger of being dropped by her insurance company.

“We got them to change their mind,” Daschle says in the commercial, which was produced by Karl Struble.

“What Tom Daschle did for me was wonderful … above and beyond the call of duty,” the woman adds.

Thune’s challenge to Daschle immediately gives the South Dakota contest the highest profile of the 34 Senate seats up this cycle.

Daschle is the only Democratic incumbent in immediate danger, although the party has five open Senate seats in the South it must defend.

Republicans face a tough re-election race for Sen. Lisa Murkowski in Alaska and a distinct possibility they will lose an open Illinois seat.

Republicans currently hold a two-seat advantage in the Senate.