MADISON Wis. (CBS 58) -- By next week, Wisconsin Senator Ron Johnson is expected to announce he's running for reelection, according to Republican sources.

The Oshkosh Republican for months has declined to say whether he's seeking a third term, but GOP sources confirm with CBS 58 he's planning to make an announcement on Monday he's running.

Once Johnson formally announces, he will break his previous pledge to only serve two terms. In past interviews, Johnson said he didn't feel pressured to decide right away because the 2022 midterms were almost a year out.

Several Democrats have already entered the race seeking to unseat Johnson. Some were quick to criticize Johnson who's poised to formally enter the race.

"Ron Johnson has been a failure and Wisconsin voters know it," said Lt. Governor Mandela Barnes. "The only people cheering Johnson's decision are the wealthy special interests and big donors who have made a killing during his time in Washington."

“Ron Johnson has failed Wisconsinites–looking out for corporate special interests and promoting conspiracy theories, not helping us,” said state Treasurer Sarah Godlewski.

Other candidates in the Senate race include Milwaukee Bucks executive Alex Lasry, Outagamie County Executive Tom Nelson, Steven Olikara, and Milwaukee attorney Peter Peckarsky.