(CNN) The ongoing coronavirus pandemic -- and President Donald Trump's uneven (at best) handling of it -- has altered the fight for the Senate majority, handing Democrats real momentum as they seek to take back control in the fall.

"What we assumed was going to be the political environment a few months ago (solid economy and record unemployment) has been turned on its head," longtime Republican pollster Neil Newhouse told me Monday. "We always assumed that it was going to be a fight to hold the Senate, and that seems more true now than ever."

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Newhouse is far from the only smart Republican voicing concerns about the way in which the coronavirus -- and the drastic economic slowdown triggered by stay-at-home and social distancing measures adopted to limit its spread -- have turned the political landscape against Trump and his party. The economic downturn, coupled with Trump's increasingly erratic public statements in regard to the virus over the past few weeks, have thrown Senate Republicans into a tizzy.

"President Trump's erratic handling of the coronavirus outbreak, the worsening economy and a cascade of ominous public and private polling have Republicans increasingly nervous that they are at risk of losing the presidency and the Senate if Mr. Trump does not put the nation on a radically improved course."

"The former vice president's emergence is part of a larger shift in prospects that has become clear in states such as Arizona, Colorado, North Carolina and even Montana and Georgia, as a surge in Democratic fundraising, along with President Trump's struggles to manage the coronavirus pandemic, have led independent analysts to upgrade Democrats' chances."

Why are Republicans so nervous? Let's look at the numbers -- and the 2020 map.

To win back the Senate majority, Democrats need to net three seats if Biden beats Trump this November, and four seats if the President is reelected. (The possibility of Massachusetts Sen. Elizabeth Warren being Biden's running mate and Republican Gov. Charlie Baker appointing an interim GOP senator in early 2021 for five months until a special election is held is also lingering out there.)

An initial look at the map suggests a major discrepancy in the numbers -- in Democrats' favor. Republicans have to defend 23 seats of their own as compared to just 12 for Democrats. (The reason? The 2014 election was a very good one for Republicans as they won nine seats and the Senate majority. All of those Senators are now up in 2020; Senate terms are six years.)

At the start of this election cycle, however, those numbers looked slightly misleading. While Republicans had major exposure in terms of the overall number of seats, only two of their incumbents (Sens. Susan Collins of Maine and Cory Gardner of Colorado) were running for reelection in states that President Donald Trump lost in 2016. Meanwhile, Democrats had to defend a seat in strongly conservative Alabama where Sen . Doug Jones (D) had won an upset special election in 2017

What's happened over the last six months or so, however, is that Democrats have successfully broadened the playing field -- putting a number of GOP seats in play even as the outlook for the incumbents in Maine and Colorado worsens.

The Cook Political Report, a nonpartisan political handicapper, now rates eight GOP seats as competitive as compared to just two (Jones and Michigan Sen. Gary Peters) for Democrats. Cook rates four Republican-held seats as pure "toss ups" (Arizona, Colorado, Maine and North Carolina) and another four (Georgia Sen. Kelly Loeffler, Iowa, Kansas and Montana) as "lean Republican."

And in truth, those ratings may actually underplay the danger to some of those GOP-incumbents.

* In Arizona, former astronaut Mark Kelly (D) has consistently led appointed Sen. Martha McSally (R) in polls and fundraising

* In Colorado, most strategists on both sides acknowledge former Gov. John Hickenlooper (D) is ahead of Gardner at this point.

* Collins appears to be in the race of her life against former state House Speaker Sara Gideon in Maine.

* North Carolina Sen. Thom Tillis (R) is having trouble breaking out of the low 40s in his matchup against former state Sen. Cal Cunningham (D).

* Montana Gov. Steve Bullock's (D) last-minute decision in March to run against Sen. Steve Daines (R) makes that, immediately, into a real race.

* Kansas Republicans are trying to force several candidates out of their Senate primary in hopes of keeping former secretary of State Kris Kobach, who lost the governor's race in 2018, from being their nominee.

Add it all up and you quickly see why Republicans are so worried. In virtually every state -- with the exception of Alabama -- their chances of either holding one of their own seats or winning Democratic seats has declined in the past two months. In some cases, like Arizona and Colorado, it has declined significantly.

While the White House remains the big prize for both parties in 190 days' time, control of the Senate isn't far behind. As the last three-plus years have shown, being in the Senate majority -- particularly if the president is of the same party -- is a massive advantage. The Republican-led Senate has confirmed both of Trump's Supreme Court picks as well as nearly 200 more judges in the lower-level federal courts (with, according to Senate Majority Leader Mitch McConnell , many more on the way).

Senate Democrats would very much like to give McConnell a taste of his own medicine with a President Joe Biden in the White House in 2021. And as of right now, they are well-positioned to do just that.