The math was never on the GOP’s side.

Republicans enjoy a 53-47 majority in the U.S. Senate, but the party’s defending 23 of the 35 seats up for grabs in the chamber this November. And while at least a half-dozen GOP seats are considered battlegrounds, Democrats are defending just a handful of vulnerable incumbents.

Two developments this month appear to be giving the Democrats a further boost.

“I do think it’s 50/50 right now. I think that Democrats' odds have improved over the past month,” predicted Jessica Taylor, who closely tracks the Senate races for the Cook Report, a leading non-partisan political handicapper.

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Scott Fairchild, executive director of the Democratic Senatorial Campaign Committee – which is the political arm of Senate Democrats – touted recently that "Democrats have built strong operations in core battlegrounds, brought more states online, and expanded the overall map to widen the path to the majority."

The presidential race will directly impact the battle for control of the Senate, with GOP fortunes closely tied to President Trump. The president’s approval rating has edged down and his disapproval rating’s edged up the past month as Americans judge Trump’s handling of the federal response to the coronavirus pandemic.

Senate Democrats, meanwhile, enjoyed a bump thanks to former Vice President Joe Biden locking up their party’s presidential nomination. Biden’s victory over populist firebrand Sen. Bernie Sanders of Vermont – who suspended his bid and backed Biden this month – was a big sigh of relief for some Senate Democrats worried that a liberal standard-bearer like Sanders would have put moderate states out of reach.

A leading indicator of the Democrats' momentum – campaign cash.

Democratic challengers in six states where GOP senators are facing challenging re-elections – Arizona, Colorado, Maine, North Carolina, Georgia and Montana – outraised the incumbents during the January-March quarter of campaign fundraising.

“I think that this first quarter should be a wakeup call to Republicans that Democrats are enthused. They are sending their money because they see that there is a real possibility of flipping the Senate,” Taylor said.

The DSCC tweeted on Sunday that “Momentum is on our side, but we can’t let up if we’re going to flip these seats in November.”

A GOP official on Capitol Hill acknowledged to Fox News that “Republican candidates and campaigns need to understand that the financial competition is only going to increase as the actual campaigns ramp up.”

The Cook Report currently lists GOP-held seats in Arizona, Colorado, Maine and Colorado as toss-ups, with Republican-held seats in Georgia, Iowa, Kansas, and Montana -- and Democratic seats in Alabama and Michigan -- as very competitive.

Democrats must net three seats to win back the majority in the chamber if they also capture the White House. If Trump wins re-election, the Democrats will need to net four seats to capture the chamber, as the vice president in the constitutional role as president of the Senate casts tie-breaking votes.

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With Biden at the top of the ticket, Senate Democrats believe that states like Georgia, Kansas, and even North Carolina may now be in reach.

“I think Biden becoming the presumptive nominee spared them having to run with Bernie Sanders on the ticket in a place that in places could have hurt them,” Taylor said. “Republicans – for better or for worse – their Senate fortunes are going to be tied a lot to what happens in the White House. A lot of these states that are critical to the presidential race are also Senate battlegrounds.”

Also helping to expand the map for Democrats is a late recruitment victory. Outgoing Montana Gov. Steve Bullock – who last year ran unsuccessfully for president – changed his mind and in early March decided to run for the Senate in his conservative state.

Republicans note that while they’re mostly playing defense, they do have “a couple of good offensive opportunities.”

Their prime target is Alabama, where Democratic Sen. Doug Jones – who won a special election in 2017 – faces an extremely difficult re-election in the ruby-red southern state. The GOP also has their eyes on Michigan, where incumbent Sen. Gary Peters repeatedly has been outraised by Republican challenger John James.

“Alabama and Michigan pose two excellent opportunities for Republican candidates to knock off Democratic incumbents,” National Republican Senatorial Committee (NRSC) communications director Jesse Hunt told Fox News. “John James has been an incredibly impressive candidate thus far, outraising Gary Peters three quarters in a row, and demonstrating the type of charisma and strong uplifting message on the campaign trail that is a good contrast to a relatively unknown politician like Gary Peters.”

The NRSC says thanks to strong fundraising, the committee’s going up with ads as early as June in some Senate battlegrounds.

A GOP official stressed that it’s time to change the narrative, arguing that “Republicans have been on the receiving end of a barrage of attack ads from dark money Democratic groups for over a year. Democratic challengers up to this point have lived a charmed life. They haven’t had much scrutiny.”