While the presidential election takes center stage, Senate races across America are heating up — with Democrats on the defensive and Republicans hoping to win at least one of six competitive races in November.

This year has one of the least favorable Senate maps for Democrats in recent memory, with Democrats defending incumbents in West Virginia, Montana, Ohio, Pennsylvania, Nevada, Michigan, and Wisconsin.

In order to take the Senate majority. Republicans need to flip two of these seats or, if they also win the White House, one. When Senator Manchin announced his retirement it all but guaranteed that Republicans would win his seat, meaning Democrats need to run the table in competitive races and win the presidency to hold a bare 50-seat Senate majority, with the tie-breaking vote going to the vice president.

As it stands, the most likely tipping-point state for a GOP Senate majority appears to be Montana, where Senator Tester is defending his seat against the GOP challenger, Tim Sheehy.

Most surveys this summer have have shown Mr. Sheehy either tied with or leading Mr. Tester, though a recent RMG Research and Napolitan Institute survey found that Mr. Tester is leading Mr. Sheehy by 5 points, the first survey to show him leading since March.

“If Montana voters knew that their vote would determine control of the U.S. Senate (it might), 57 percent would vote for the Republican, and just 39 percent would vote for the Democrat,” the president of RMG research, Scott Rasmussen, said of the poll. “Seventeen percent of Tester’s voters would prefer GOP control of the Senate.”

Mr. Tester is one of three Democratic incumbent senators defending seats this year expected to skip the Democratic National Convention next week. Senator Brown of Ohio and Senator Rosen of Nevada are also expected to not attend.

Aside from Mr. Tester, though, Democrats appear to be outrunning the top of the ticket in their Senate races, at least for now.

In Ohio, a recent ActiVote survey found Mr. Brown led businessman Bernie Moreno by 5 points, 53 percent to 48 percent. In neighboring Pennsylvania, an Emerson College survey found Senator Casey leads businessman David McCormick by 5 points. 

In both of these states the Democratic incumbent has maintained consistent polling leads this cycle.

In Nevada, a recent survey from Redfield and Wilton Strategies and the Telegraph found Ms. Rosen leads the GOP nominee, Sam Brown, by 3 points in a multi-candidate field.

In Michigan, a survey by Fabrizio, Lee and Associates, Impact Research, and AARP found Congresswoman Elissa Slotkin leading the GOP nominee, Mike Rogers, by 3 points.

In Wisconsin a TIPP Insights and American Greatness survey found Senator Baldwin with a 7-point lead over the GOP nominee, Eric Hovde.

In a recent analysis by political scientist Jackson Hamilton, writing at Sabato’s Crystal Ball at the University of Virginia Center for Politics, two Senate races in particular stand out for Democrats: Pennsylvania and Wisconsin. That’s because in both these races the Democratic incumbent is polling above 47 percent on average.

“This is notable because, in the competitive senate elections from 2010 to 2022, not a single incumbent who received at least 47 percent in the Aug. 1 polling average lost reelection,” Mr. Hamilton wrote. “While it is certainly still possible that the standing of Casey and Baldwin could weaken between now and Election Day, historical precedent suggests that they are in a strong position to win reelection.”

Mr. Hamilton also notes that Senator Cruz is also polling above 47 percent support on average in Texas, suggesting that he will likely win re-election. Mr. Brown and Ms. Rosen are also polling at nearly 47 percent support.

As it stands, polling suggests an unexpectedly optimistic picture for Democrats heading into the final stretch of campaign season, given the map. 

According to a political spending watchdog, AdImpact, Democrats have also outspent Republicans in every competitive Senate race this year, with the parties spending a combined $973 million. In total, Democrats have dedicated $570 million to their Senate efforts across seven states, while Republicans have dedicated $403 million.