Ron Johnson, G.O.P. Senator From Wisconsin, Will Seek Re-election

WASHINGTON — Senator Ron Johnson, a Republican from Wisconsin who over the last year has become the Senate’s leading purveyor of misinformation about elections and the coronavirus pandemic, announced Sunday that he would seek re-election to a third term.

Johnson, 66, had promised to step down after two terms, but opened the door for a third shortly before 2020’s presidential election.

His entry into this race is sure to draw huge attention to Wisconsin, which is a tightly divided political battleground where Gov. Tony Evers, a Democrat, faces his own difficult re-election bid in a race that may determine control of the state’s election systems ahead of the 2024 presidential contest.

“Today, I am announcing I will continue to fight for freedom in the public realm by running for re-election,” Mr. Johnson wrote in an essay published Sunday in The Wall Street Journal.

Mr. Johnson’s decision follows an announcement Saturday from another Senate Republican weighing retirement, Senator John Thune of South Dakota, that he would seek a fourth term.

The Wisconsin Senate contest is expected be among the most closely contested in the country. Democrats loathe Johnson and have attracted double-digit fields of challengers to him in the general elections. Local Democrats have been raising money for almost a year to create a turnout machine for 2022’s midterm elections.

When Mr. Johnson first entered politics in 2010 as a self-funding chief executive of a plastics company founded by his wife’s family, he defined himself as a citizen legislator in contrast with Senator Russ Feingold, a Democrat who had been in public office for 28 years. Mr. Johnson was carried into office by that year’s Tea Party wave, then beat Mr. Feingold again in 2016 as Donald J. Trump became the first Republican presidential nominee to win Wisconsin in 32 years.

All along, Mr. Johnson pledged to serve no more than 12 years in the Senate, but he began to privately reconsider after the 2018 elections, when Democrats took back control of the House of Representatives and won narrow victories in Wisconsin’s statewide elections. He wrote Sunday that when he made reiterated his two-term pledge during his 2016 race he didn’t anticipate “the Democrats’ complete takeover of government and the disastrous policies they have already inflicted on America and the world.”

Now, he is the leader of Wisconsin Republicans as well as the lone G.O.P. As the only G.O.P. in Wisconsin, Johnson was suddenly the leader of Wisconsin Republicans. They argued that if Johnson did not run again, it would put at risk a seat that could tilt the balance in the Senate in 2023.

Mr. Johnson wrote that he was seeking a new term because “I believe America is in peril,” adding: “Much as I’d like to ease into a quiet retirement, I don’t feel I should.”

This year, Mr. Johnson has been at the forefront of the two strongest strains of misinformation coursing through the Republican Party — false claims about election administration and public health.

In the days after the 2020 election, he challenged Joseph R. Biden Jr.’s victory. During a Senate hearing in February, he read into the record a report that falsely suggested the Trump-inspired Jan. 6 assault on the Capitol had been instigated by “fake Trump supporters.” In November, he began urging Wisconsin’s Republican state legislators to seize control of federal elections in the state, arguing that they could do so without the governor’s approval, despite decades-old rulings from the U.S. Supreme Court and the Wisconsin Supreme Court that say otherwise.

Johnson is the Republican leader who has made the most false claims about coronavirus vaccines and Trump. Johnson has stated that he won’t get vaccinated and has discredited Covid-19 treatments. He also declined to encourage others seeking the vaccines. He claimed in December that gargling with mouthwash could stop the transmission of the virus. This assertion drew a rebuke form the manufacturer Listerine.

While Mr. Johnson’s false claims have picked up pace recently, they go back years. During his 2010 campaign, he said that climate change was caused by “sun spots” and that excess carbon dioxide in the atmosphere “helps the trees grow.”

Mr. Johnson is a Wisconsin Democratic’s strongest fund-raising boogeyman, and a figure many view as an embarrassment for the state in the same vein as Senator Joseph McCarthy.

“He’s an active menace to American democracy, a threat to public health and an economic saboteur of the middle class,” said Ben Wikler, chairman of the Democratic Party of Wisconsin. “His sole redeeming quality in public life is that in 2022, he’s going to inspire Democrats to organize and turn out.”

Lt. Governor is one of the Democrats who are vying for Mr. Johnson’s support. Mandela Barnes; the state treasurer, Sarah Godlewski; Tom Nelson, the executive of Outagamie County, which includes Appleton in Wisconsin’s Fox Valley; Alex Lasry, an executive with the Milwaukee Bucks basketball team, and a handful of lesser-funded candidates. None of the Democratic challengers in the state are as well-known as Mr. Johnson.