Sen. Tim Johnson (D-S.D.) announced Friday that he is running for re-election in 2008 — the clearest sign yet that he continues to improve from a Dec. 13 brain aneurysm that kept him out of the Senate for nine months.

Johnson’s official entry into the political fray should give Republicans some license to go on the attack after nearly a year of treating the incumbent with kid gloves because of the nature of his illness and the sympathy it engendered for him in South Dakota.

But it may not matter, as the GOP has been unable to recruit popular Gov. Mike Rounds — or any other top-tier Republican — into the race thus far. South Dakota GOP Executive Director Max Wetz said there are still some top-tier candidates considering a bid, but he declined to provide any names on Friday.

“The unfortunate reality for Republicans is the only guy we have capable of giving Johnson a run for his money is Mike Rounds,” said one GOP strategist based in Washington, D.C. “If Johnson’s health continues to improve and Rounds doesn’t have a change of heart, South Dakota could be a pretty steep hill to climb for Republicans looking for a Senate seat in 2008.”

The National Republican Senatorial Committee declined to comment Friday on candidate recruiting or other plans for targeting Johnson in 2008.

To date, the only announced Republican candidates are state Rep. Joel Dykstra and businessman and state GOP outsider Sam Kephart, who finished the third quarter fundraising period with $37,000 and $350 in the bank, respectively. Johnson closed the period with more than $2 million on hand on the strength of $454,000 raised.

Rounds spokesman Mitch Krebs said Friday that the governor has no plans to run for Senate in 2008, but he declined to elaborate further or specifically close the door on a Johnson challenge. Republicans familiar with South Dakota say Rounds is risk-averse and unlikely to run against Johnson, a two-term Senator who is politically formidable and remains popular in his own right.

Other prominent Republicans who could be well-positioned to take down Johnson also are taking a pass this cycle, with many of them focused on running for governor in 2010, when Rounds’ second term will expire.

They include Lt. Gov. Dennis Daugaard; Kelby Krabbenhoft, a possible self-funder who is president and CEO of Sioux Valley Hospitals & Health System; and state Senate Majority Leader Dave Knudson.

Daugaard already has announced for the 2010 gubernatorial race. Knudson is still considering the 2010 gubernatorial race, while Krabbenhoft has announced that a run for federal office in 2008 is off the table.

Some Republicans argue it is too early to assume the GOP will be unable to field a top-tier candidate against Johnson. They contend that potential Republican challengers have been waiting to see what Johnson was going to do before making their own decisions about whether to run.

“There are a couple of people that are still considering the race,” Wetz said. “They will weigh this new development and hopefully make decisions pretty quickly.”

With the Johnson question now answered, some shoes might drop in the GOP’s favor, according to some party strategists. In a state that routinely votes heavily Republican for president — and with the possibility that Sen. Hillary Rodham Clinton (N.Y.) might top the Democratic ticket — Republicans have an argument to make that Johnson could be vulnerable.

In fact, now-Sen. John Thune (R-S.D.) knocked off then-Senate Minority Leader Tom Daschle (D-S.D.) in 2004, which was a presidential cycle.

But Democrats say Johnson is a great candidate who fits with South Dakota’s voters. They expect him to win re-election in 2008 and are thrilled that he chose to run, although they are not taking anything for granted.

“Tim Johnson’s recovery has been an inspiration to his colleagues and to the people of South Dakota. He’s an outstanding Senator and will be a great candidate,” Democratic Senatorial Campaign Committee spokesman Matthew Miller said.

Johnson campaign spokesman Steve Jarding said the Senator chose Friday to make his re-election bid official because he had been receiving numerous inquiries about his plans for 2008 and wanted to provide the certainty his supporters and constituents desired.

Jarding said most of the questions were motivated by Johnson’s illness and whether he has really recovered sufficiently to handle running for re-election. Johnson resumed work in the Senate for the first time this year just after Labor Day.

Jarding said a more formal announcement will come early next year.

“Clearly there were some people that weren’t certain, because we were getting questions,” Jarding said. “In fairness, because of Tim’s health there were some questions in the last few months. But everything does seem to be going very well and the doctors have said it’s a green light to go.”

John McArdle contributed to this report.