Three charts that show how Republicans are losing their chance to keep the Senate

"Democratic chances of winning control of the chamber have improved significantly (in the last nine months)," write Nathan Gonzales and Jacob Rubashkin of Inside Elections. And in their latest ratings update, The Cook Political Report says Democrats are now a slight favorite to win the Senate majority.

There are 35 Senate seats at stake this November, 12 of which are held by Democrats and 23 by Republicans. That gives the Republicans an inherent advantage. But since November of last year, that strong Republican defense has slowly been eroding, opening up the possibility for Democrats to net enough seats for a majority takeover. The number of states rated "Solidly Republican" by The Cook Political Report has shrunk from 13 to 10.

Democrats now have 47 seats in the Senate, including two independents who caucus with them. To win the Senate majority, they would need to defend those 12 seats and pick up an additional four this November -- or three, if former Vice President Joe Biden, a Democrat, were to win the White House and could send his vice president to serve as a tie-breaking vote. Unlike in 2018, when Democrats were mostly defending seats, many of the Senate elections this year are in states that voted for President Donald Trump and are seats defended by Republican senators.

Since November 2019, Republicans in Cook's forecast have made headway in deeply red Alabama and Mississippi but have lost ground in nine other states. Those include not just battleground states like North Carolina and Arizona, but also Georgia, Montana and Kansas, which have reliably voted GOP for decades. Every seat downgraded by forecasters presents another pathway for Democrats to win the 16 races needed for a majority.

Cook determines its ratings by talking to in-state and national sources, looking at both public and private polls, and by judging candidate quality and fundraising, explained Cook's Senate editor Jessica Taylor in an email. A race rated as "Solid" means it's not considered competitive nor likely to become closely contested. A "Solid" race downgrades to "Likely" if that race gets an unusually strong opposition candidate or closer-than-expected polling. And "Likely" becomes "Lean" if that race becomes increasingly competitive but one party still holds an advantage.

Democrats will still have to defend a US Senate seat in Alabama, which voted for Democrat Doug Jones in the 2017 special election over Republican Roy Moore, an exceptionally flawed candidate. Democratic Sen. Gary Peters will also have to defend his seat in Michigan, a Trump-voting swing state targeted by Republicans as a pickup. Since November, Michigan's "Leans Democratic" rating is unchanged but Jones' race in Alabama was bumped up from "tossup" to "Leans Republican."

Despite the GOP's inherent advantages going into this race, a series of events have not gone the party's way this election year. Democrats are running strong candidates and matching or out-fundraising their rivals even in traditionally red states such as Alaska, Montana and South Carolina. Montana, which Cook rated Solidly Republican as recently as March, became a Tossup after its popular governor, Steve Bullock, won the Democratic primary. "(Bullock) was virtually the only Democrat who could have put this race into play," Taylor said.

Republican senators also can't escape the unique circumstances of 2020. "In a presidential election year, even down-ballot elections are often driven by the top of the ticket, and that's especially true with a president that has such an iron grip on his party," Taylor said.

The President's poll numbers have wilted over the past few months amid his responses to the coronavirus pandemic and the Black Lives Matter movement, creating an increasingly toxic environment for any Republican candidate. In Georgia, for instance, both Republican senators are up for reelection this year. Representing a state with shifting demographics and particularly hard-hit by coronavirus, the senators' Cook ratings have sunk from Likely Republican to Leans Republican (Sen. Kelly Loeffler) and Tossup (Sen. David Perdue) since the pandemic began.

Two other leading election forecasters have reached similar conclusions. Inside Elections and the Center for Politics Larry Sabato's Crystal Ball have each reduced the number of Republican seats rated "Solid" or "Safe" over the past nine months.

While several red state ratings have weakened, a Democratic takeover of the Senate still relies on Tossup and Leans Republican states tilting in their favor. For Senate Democratic leader Chuck Schumer to have a chance of becoming the Senate majority leader, he'll have to hope the political climate doesn't boomerang between now and November with the same fervor as in the last few months.