Weaker Republican opponents in some states have also played a role: In Arizona, GOP nominee Blake Masters’ struggles since winning the primary last month have moved the state from “Toss Up” to “Lean Democratic” — a reflection of Democratic Sen. Mark Kelly’s growing lead over Masters.

The Arizona shift leaves four “Toss-Up” Senate races — two currently controlled by each party. If neither party wins one of the races where the opposition has an advantage now, Republicans would need to win three of the four “Toss Up” races to wrest control of the majority.

Here are the four key elements that have brought control of the Senate closer to a coin flip:

The improving political environment for Democrats

On July 1, the polling average of President Joe Biden’s approval rating stood at 39 percent, according to FiveThirtyEight. As of Labor Day, it’s now 43 percent — still low, but enough of an improvement to raise Democrats’ fortunes.

Democrats have also erased a 2-point deficit on the generic congressional ballot over the same time period. They now lead Republicans by a point, according to FiveThirtyEight.

Though modest, that swing boosts Democrats’ Senate odds, since the battlefield is already centered in states Biden won in the 2020 presidential election. Biden carried each of the four remaining “Toss-Up” states — Georgia, Nevada, Pennsylvania and Wisconsin — in addition to the three currently rated “Lean Democratic”: Arizona, Colorado and New Hampshire.

If the GOP holds each of the “Lean Republican” states, it needs to win at least three Senate seats in Biden-won states — a tougher task if public opinion of the president’s job performance improves further over the next nine weeks.

Abortion as the pivot point

The biggest speed bump in Republicans’ march to the Senate majority has been the Supreme Court’s decision reversing nearly 50 years of federal abortion rights.

Between Democrats’ slight polling recovery, the party’s overperformance in recent special congressional elections and the results of a ballot initiative in solid-red Kansas, the high court’s Dobbs decision has delivered a jolt to the midterm landscape, at least temporarily.

It’s also scrambled the Senate landscape and sent candidates like Masters looking for a new message in the face of broad public disapproval of the court’s decision.

But Democrats still face this uncomfortable reality: The economy and inflation are still issue No. 1 for voters, and it’s still the message that shows up in most of both parties’ campaign ads.

Republicans’ candidate struggles

Masters is underfunded, lagging behind his Democratic opponent and getting swamped on the airwaves. Democrats have savaged him over his past statements, sending his negatives spiking — all after emerging from a fractious primary.

That’s bad enough for Republicans. Even worse is that Masters’ plight is not unique to GOP Senate challengers.

In addition to Masters, that could also be true of Mehmet Oz in Pennsylvania, or J.D. Vance in Ohio, or Herschel Walker in Georgia (though Walker’s primary victory was more decisive).

If Republicans have a candidate problem, it’s most acute in Arizona, where the top GOP super PAC, Senate Leadership Fund, canceled millions in planned advertising, diverting the money elsewhere, including to Ohio.

But Oz in Pennsylvania has also struggled: His image ratings are badly upside-down, though Republicans are hoping to narrow the race by attacking the Democratic nominee, Lt. Gov. John Fetterman, as soft on crime.

And though Walker is in better shape in Georgia, allegations of domestic violence and fabricated elements of his resume are a staple of Democratic advertising, which aims to disqualify the former football star in the minds of potential swing voters.

Democrats (slightly) expanding the map

The GOP’s tension over its candidates is only half the story. Democrats are also opening new lanes in their fight to retain the majority, largely because their candidates have so much money.

Since the May primary, Democratic Rep. Tim Ryan has spent $12 million in Ohio — a significant sum that dwarfs Vance’s $300,000 (Vance and the National Republican Senatorial Committee have also run $1.7 million in coordinated ads). That’s a big reason why Senate Leadership Fund is diverting money from Arizona, where their candidate now trails, to Ohio, a state that should stay red — that is, as long as the GOP isn’t massively outspent.

The same is also true of Florida, where Democratic Rep. Val Demings’ $22 million in ads outpaced GOP Sen. Marco Rubio by a roughly 5-to-1 marginleading up to the August primary, according to data from the tracking firm AdImpact. And in North Carolina, Democratic nominee Cheri Beasley has spent $6.3 million on ads, while GOP Rep. Ted Budd has relied almost exclusively on the NRSC to respond.

All three races — Florida, North Carolina and Ohio — are rated “Lean Republican,” meaning the GOP is still favored. But Republicans are being forced to spend real money to defend those seats. Senate Leadership Fund has booked $27.7 million in ads in North Carolina and $27 million in Ohio.

While Democrats have broadened the Senate map, Republicans have struggled to do the same.

Republicans had high hopes for Tiffany Smiley’s well-funded challenge against Sen. Patty Murray (D-Wash.) earlier this summer, but the results of the Aug. 2 all-party primary — in which Murray received 52 percent of the vote and other Democratic candidates took a few points more of the total vote — suggested the contest is unlikely to materialize. The race remains “Likely Democratic.”

Similarly, the GOP has touted its nominee in Colorado, Joe O’Dea. But his race against Democratic Sen. Michael Bennet hasn’t become a top target either — it’s rated “Lean Democratic.”

A sea change that puts both blue-state seats in play would offer Republicans a wider path to the majority — but also would likely mean they’ve pulled ahead in states like Georgia and Pennsylvania. That hasn’t happened yet.