Only a few months ago, at the beginning of the summer, Senator Johnson’s seat in Wisconsin was considered safe. Today, it’s one of America’s most competitive.

A Marquette University Law School poll out Wednesday shows the incumbent Republican and his Democratic challenger, Mandela Barnes, all but tied at 48 percent to 49 percent.

Mr. Johnson is campaigning on dissatisfaction with “radical gender theory” being taught in schools and high crime, conspiracies about the Biden administration’s handling of the Covid pandemic, and alleged bias against conservatives on social media platforms.

Mr. Barnes is emphasizing abortion rights, “protecting democracy,” and “fighting inflation and lowering taxes.” He has already begun to campaign against Senator Graham’s proposed national abortion ban.

Both candidates have emphasized the role that the seat would play in giving Democrats a 51-seat Senate majority if Mr. Barnes were to win. 

Wisconsin is no stranger to close elections — President Biden defeated President Trump in the 2020 election by only 19,000 votes there — but the race is unexpectedly weighing down national Republicans, who would rather deploy resources elsewhere.

A political scientist at John Jay College, Brian Arbour, says that in an ideal world, Republicans would “like to put more bucks into states like Colorado.”

“Ron Johnson is an unusual person in that he’s always run better than expectations,” he tells the Sun, with the caveat that Mr. Johnson continues to be perceived by some as “not the best candidate.”

In the 2016 Senate race, Mr. Johnson defeated the incumbent senator, Russ Feingold, outrunning the polls by nearly 6 points and snatching victory with 50 percent of the voter to Mr. Feingold’s 47 percent.

Analysts at Decision Desk HQ are betting that Mr. Johnson will do it again in 2022. Betting on a polling miss is an unreliable strategy, though, and the forecasters are divided on the likelihood of Mr. Johnson pulling out another win.

In an ideal world the national GOP would only need to fund races where it can flip seats, but Mr. Arbour says that “you’d frankly expect Republicans to put more resources into Wisconsin,” given the need to defend their seats in an evenly divided Senate.

The Wisconsin race could potentially attract some of the money that had been committed to the Senate race in New Hampshire. A political action committee closely aligned with Minority Leader McConnell, the Senate Leadership fund, committed $23 million to the Granite State race before the primary there.

An associate editor at Sabato’s Crystal Ball, Miles Coleman, speculates that results of that primary might make Mr. McConnell reconsider his fiscal commitment to the state.

An Army brigadier general, Donald Bolduc, defeated the New Hampshire state senate president, Chuck Morse, with only 37 percent of the primary vote.

To Mr. Coleman, the nomination of Mr. Bolduc strengthens the position of Senator Hassan, the incumbent Democrat, who has “lucked out” this year by avoiding stronger Republican opponents.

“I’m interested to see what happens to that $23 million from the Senate Leadership fund now that it’s Bolduc,” Mr. Coleman says. “He is a Trumpier candidate that wouldn’t necessarily be Mitch McConnell’s choice.”

Mr. Bolduc is already attempting to reverse positions he staked out in the primary, such as his claim the 2020 election was stolen. Now, he says that “unfortunately, President Biden is the legitimate president.”

With the GOP’s chances in New Hampshire dwindling and Wisconsin in play, the path to 51 Republican Senate seats is narrowing.

The race in Pennsylvania alone limited the opportunity for a Republican majority. A poll out Thursday morning found Lieutenant Governor John Fetterman, a Democrat, running five points ahead of his Republican opponent, Mehmet Oz.

The Pennsylvania seat is currently held by a Republican, so a loss in the Keystone State would mean that Republicans need to flip two seats and protect all of their other contested seats like Wisconsin, Ohio, or North Carolina to gain control of the Senate.

Mr. Arbour said the most viable path to a Republican majority in the Senate runs through the Sun Belt — Georgia, Arizona, and Nevada. That path has its own challenges.

In Nevada, the Republican state attorney general, Adam Laxalt, is seen as a strong candidate, with a good chance of defeating Senator Cortez Masto, the incumbent Democrat. Republicans, however, would need to win two of these Sunbelt states for a majority.

In Georgia and Arizona, Republicans need to propel a pair of lackluster candidates — retired football star Herschel Walker and venture capitalist Blake Masters — over the finish line.

“There’s a more sort of specific problem in that the Republicans don’t have very good candidates,” Mr. Arbour says. “Very few of them have won elected office let alone statewide office.”

He says that in these states, inexperienced candidates have created “target-rich environments for opposition researchers.” The result is Republican Senate hopefuls underperforming the generic ballot in key battlegrounds.

If Republicans were to “just pick a member of Congress out of a hat in any of these states,” they would have “basically run as well as the generic ballot,” Mr. Arbour said.