It would benefit McCaskill if the two Republicans beat each other up in the primary and left a weakened GOP candidate in the general election.

The first-term senator's job approval numbers didn't crack 50 percent in any of the four publicly released polls conducted this year. But she had managed to keep her poll numbers from plunging during the rough period for Democrats from late 2009 through 2010 — even while Missouri voters turned strongly against President Obama and Democrats — by appealing to centrist voters and maintaining an image of integrity.

But that image was shaken when she admitted in March that she owed $300,000 in back taxes on a private plane she owned with her husband, and that she had used taxpayer money to fly the plane on a political trip. She tackled the issue head on, apologizing to voters, paying the back taxes, and selling the plane.

Republicans attacked her on that issue as well as her votes with Obama. "Claire McCaskillClaire Conner McCaskillManchin used Heimlich manuever on McCaskill during caucus luncheon Nail manufacturing exec who voted for Trump blames him for layoffs, asks Democrat for help The American economy is stronger than ever six months after tax cuts MORE will never be able to raise enough money to make Missourians forget her disastrous ethical problems or the fact that she has routinely put her loyalty to President Obama over the best interest of her state," said National Republican Senatorial Committee press secretary Chris Bond. "Whether it’s failing to pay more than $300,000 in taxes on her private plane or casting the crucial 60th vote to pass ObamaCare, McCaskill’s record puts her at odds with middle-class Missourians."

Democrats in Missouri emphasized the strong fundraising numbers, however.

"Claire's focus on commonsense solutions has won her broad support across Missouri and respect throughout the country," said Missouri Democratic Party spokeswoman Caitlin Legacki. "We're grateful for the outpouring of support we have received and are confident that we will have the resources to be successful."

The election will be one of the most closely watched and hard-fought Senate campaigns in the country. McCaskill, a top GOP target next year, will need to have more fundraising quarters like this one if she hopes to maintain her seat.

-- This story was updated at 6:43 p.m.