An incumbent United States senator usually doesn’t have a difficult time winning re-election -- when his party's presidential nominee is leading his state by an overwhelming margin. 

But that's not the case in Missouri where Republican Sen. Roy Blunt finds himself barely ahead of Democrat Jason Kander despite GOP presidential nominee Donald Trump’s lead reaching almost double digits in the state. The race is one of a handful that could decide which party controls the Senate next year.

"Folks in Missouri know we need to change Washington,” Kander said at an election eve rally in St. Louis. “We know we need a new change in leadership to get it done.”

That outsider message has cracked the door open for Missouri’s secretary of state and hurt Blunt, 66, who has spent the last 20 years in Washington, 14 in the House and six in the Senate.

Both Kander, 35, and Trump have promised to rebuild the middle class. They also rail against millionaires, billionaires and special interests who they believe control politicians.

When asked by Fox News about his outsider message and its similarity to Trump’s, Kander replied, “I don't happen to think that Donald Trump is qualified to be president, but I understand why people want to shake up that conversation."

The Afghanistan War veteran burst into the national spotlight earlier this year when he aired an ad defending his position supporting background checks for gun buyers by assembling an AR-15 rifle blindfolded. 

He’s also separated himself from some Democrats and the White House by opposing the Iran nuclear deal, supporting keeping open the military prison at Guantanamo Bay and supporting a federal balanced-budget amendment. 

Kander has hit Blunt hard, painting the senator as a Washington insider with a family full of lobbyists. Blunt’s wife and three of his children are lobbyists.

Blunt’s responded to the attacks by aligning Kander with Democratic presidential nominee Hillary Clinton.

The Republican Conference vice chairman is pitching himself as a crucial check on her power if Clinton wins, while also stressing the importance of keeping the Senate in Republican hands for two more years.

“Every legislation in the Senate is determined by the majority,” Blunt told volunteers at a campaign office outside of St. Louis on Monday.

He continued to warn that conservatives risk losing the Supreme Court for a generation if Republicans lose the White House and control of the Senate. Blunt pointed to crucial decisions on abortion and immigration that could be affected by the court.

When asked about his lower poll numbers in the state compared with Trump, the Missouri senator stated he always thought the race would be tough.

“We have two statewide public officials that are Republican out of eight so I always assume that this is going to be a tough thing,” Blunt said.

Republicans were always playing defense this cycle to keep control of Congress’ upper chamber, which they won two years ago, but were not expecting the Missouri race to be this competitive.

When it became clear it would be a close race, money began pouring in from outside sources.

Outside groups have spent about $45 million on the race so far. That’s more than the almost $26 million raised by both campaigns combined.

The Republican Senatorial Campaign Committee so far has spent more than $4 million on the race. Its Democratic counterparts have spent almost $7 million.

Privately, some Republicans are frustrated they have had to spend so much money attempting to defend the seat in the first place, amid concerns Blunt waited too long to take Kander seriously. One Republican said if the party does lose the control of the Senate, Blunt “has blood all over his hands.”

Jason Donner is a Fox News Producer based in Washington, D.C. Follow him on Twitter: @jason_donner