Democratic Senate candidates, spurred by anger at U.S. President Donald Trump and hope that the party could take back the Senate, have posted massive second quarter fundraising numbers this month, distressing Republicans who now fear November's election could be devastating.

The hauls are giving some Republicans déjà vu, reminding them of the 2018 midterms, when dozens of Democratic challengers across the country consistently outraised Republican incumbents. That fundraising superiority led Democrats to take back the House.

Some of the most substantial numbers have come in deep red states that Trump won handily in 2016. In South Carolina, Democrat Jaime Harrison announced on Tuesday that his campaign raised nearly US$14 million in the second quarter, building on the $7.3 million he raised in the first quarter. Republican Sen. Lindsey Graham has not yet released his second quarter fundraising totals, but Harrison outraised Graham's $5.7 million in the first three months of 2020.

And in Montana, Democratic Gov. Steve Bullock announced he had raised $7.7 million in the second quarter, after raising about $3.4 million in the first quarter, despite entering the race with just weeks left in the quarter. His Republican opponent, Sen. Steve Daines, has not yet released second quarter fundraising totals, but Daines only raised about $1.3 million in the first quarter.

"ActBlue gets too much credit for Democrat fundraising success," said a senior Republican strategist, pointing to Democrats' potent small-dollar fundraising apparatus. "Donald Trump is far and away the greatest fundraising tool in the Democrats' arsenal."

The Republican concerns are less about what the money means for this moment, and more about how growing war chests among Senate Democratic candidates will directly lead to nonstop ad campaigns in the fall, when voters are paying closer attention to these key Senate race. That scenario played out in in 2018, where Democrats dominated the airwaves in House races ahead of the November elections because of the fundraising superiority they built earlier in the year.

This year, the Senate map tilts toward Democrats. Republicans, with special elections in Georgia and Arizona, are defending 23 Senate seats, compared to only 12 seats for Democrats. If Democratic presumptive nominee Joe Biden wins the presidency in November, Democrats need to win three seats to take control of the legislative body, and four if Trump wins another term in the White House. Cook Political Report has five Republican seats as "toss ups," compared to only one Democratic seat -- Sen. Doug Jones in Alabama -- that is rated "lean Republican."

While fundraising numbers from Harrison and Bullock are striking, the most concerning, Republicans said, are the those sizable hauls in places like Maine and North Carolina, two states where Republican incumbents keeping their seats will be central to the party holding the Senate.

In Maine, Democratic candidate Sara Gideon announced that her campaign had raised more than $9 million in the second quarter. Gideon had raised about $7.1 million in the first quarter, dwarfing the $2.4 million Republican incumbent Sen. Susan Collins raised in the first quarter. Collins has not released her second quarter numbers.

And in North Carolina, Democrat Cal Cunningham announced a $7.4 million second quarter haul, building on the about $4.4 million he raised in the first quarter of 2020. His Republican opponent, Sen. Thom Tillis, has not yet released second quarter fundraising totals, but he raised about $2.1 in the first three months of the year.

"Democratic Senate candidates have had strong grassroots fundraising all cycle, and these latest record-breaking numbers reflect the growing interest in these Senate battlegrounds and an unprecedented motivation to hold Republicans accountable," Lauren Passalacqua, communications director at the Democratic Senatorial Campaign Committee, said.

Even when Democratic challengers haven't posted huge hauls, they are still competitive with their Republican opponents. In Georgia, Democratic candidate Jon Ossoff announced he raised $3.5 million in the second quarter, including nearly $2.4 million more has come in since he won his primary on June 10. His opponent, incumbent Sen. David Perdue, has not yet announced his second quarter fundraising totals. But Perdue reported raising about $1.7 million in the first three months of the year and enjoys a notable $9.4 million in cash on hand as of May 20, when he filed his pre-primary report.

"Americans are clearly exhausted from the divisive politics and failed leadership of President Trump and his Senate Republican enablers," said Matt Corridoni, a spokesman for Senate Majority PAC, the primary super PAC supporting Democratic Senate candidates. "This groundswell of support is a testament to a deep bench of Democratic candidates who are running strong campaigns across the battlefield talking about what matters most to help working families go forward in a struggling economy."

The strong fundraising hauls from Senate Democratic candidates also have the potential to expand the map. With ample resources on the campaign side, Democratic super PACs and other outside groups will be able to spread money around more races, without the pressure of buttressing underfunded candidates in key states.

Jesse Hunt, the top spokesman for the National Republican Senatorial Committee, said that Democrats will "need to spend every penny to defend records that are disqualifying in the eyes of mainstream voters who will decide the outcome in key Senate races."

"Personal scandals and a party rallying around a socialist agenda are problems money can't solve," Hunt added.

Still, the sizable Democratic fundraising in some races has put less-than-stellar hauls from other Democrats in stark contrast.

In Texas, Democrat MJ Hegar announced she raised $1.7 million in the second quarter. Republican incumbent John Cornyn has not yet released second quarter fundraising totals, but Cornyn raised about $2.7 million in Q1 and had nearly $13 million in cash on hand as of March 31.

Hegar's small haul is particularly striking given that when former Texas Rep. Beto O'Rourke ran for Senate against Republican Sen. Ted Cruz in 2018, his second quarter haul was more than $10 million.

The strong Democratic fundraising extends beyond Senate race, too.

Former Vice President Biden, after initially struggling to keep up with Trump's strong fundraising, raised $141 million in June, outpacing the $131 million the Republican President's campaign pulled in. It was the second month in a row where Biden outraised Trump: The Democrat raised $80.8 million in May, compared to Trump's $74 million haul.

Some Democratic campaigns that are known for strong fundraising have yet to put out their numbers.

In Arizona, Democrat Mark Kelly raised $11 million in the first three months of the year, nearly doubling the $6.2 million his opponent, Republican Sen. Martha McSally, raised in the same time period.

The financial struggles, especially in the Senate races, is clearly worrying Republicans, and fundraising emails from those incumbent senators are getting more dire.

"In order for me to beat my SURGING opponent and defeat Chuck Schumer's Senate take-over plan -- I need 150 Trump supporters to step up and donate to my campaign BEFORE MIDNIGHT," Montana's Daines wrote in an email to supporters on Tuesday. "Everything is on the line this election and the next couple hours could make or break the outcome."