Sen. Tim Johnson, D-S.D., said he will make a “formal announcement later next year” about his re-election in a carefully worded statement from his office released Thursday morning.

“As in past campaigns, I will make my formal announcement later next year,” he said. “But I feel great, still have work to do, and I fully intend to put together a winning campaign in the weeks and months ahead.”

Former Gov. Michael Rounds, a Republican, announced Thursday that he’s jumping into the Senate race.His candidacy immediately makes the race competitive, and Roll Call rates it as a Tossup.

“I am here today to ask the people of South Dakota for their support and to allow me to work for them as their United States Senator in 2014,” Rounds said in a statement.

Johnson remains the only Democrat to hold statewide office in South Dakota, a state that President Barack Obama lost by 20 points. If Johnson were to decide not to run, Rounds would have the upper hand in the Senate race.

Also in his statement, Johnson praised Rounds’ “refusal” to take Grover Norquist’s famous pledge not to raise taxes under any circumstances.

“While I’m sure that Mike and I will have some policy disagreements — which is to be expected — I am more than willing to give credit where it is due,” Johnson said. “And the fact that Mike realizes that it is irresponsible to take tax increases off the negotiating table is absolutely to his credit.”

The Norquist pledge looms large over the Capitol Hill debate to avert the fiscal cliff. But Democrats might have an ulterior motive for mentioning Rounds’ resistance to the tax pledge.

Since Rounds announced an exploratory committee weeks ago, Democrats started whispering about a potential primary challenge for the former two-term governor. No potential primary names have surfaced yet, so that might be wishful thinking on behalf of Democrats.

Nonetheless, in the past two cycles, Senate Republicans failed to pick up five seats because of lackluster primary victors.