Internal polling on both sides of the Missouri Senate race show Republican incumbent Roy Blunt Roy Dean BluntSocial media platforms put muscle into National Voter Registration Day Senate GOP faces pivotal moment on pick for Supreme Court This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda MORE, a longtime member of the GOP congressional leadership, is in the toughest race of his career.

If he goes down in Republican-leaning Missouri, where Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE’s unfavorable rating is sky high, he’ll likely take down the Senate GOP majority with him, strategists in both parties say.

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The prospect was unthinkable earlier this year, given Missouri’s decidedly Republican leanings and the incumbent’s long and successful political career.

Blunt, the vice chairman of the Senate Republican Conference, was easily elected to the Senate in 2010. Before that he served as House majority leader, narrowly missing a chance to become Speaker, and his son, Matt, served as governor of Missouri.

But his 20 years in Washington and his family’s political connections have become a liability in an election year where many voters are angry at Washington and entrenched special interests.

Burson Snyder, a spokeswoman for Blunt, said her boss “has led in every internal poll this cycle,” but other Republican strategists familiar with internal GOP polls say his lead in the low single digits at best.

And one GOP strategist familiar with the race said internal Republican polling last month showed Blunt trailing his Democratic opponent, 35-year-old Secretary of State Jason Kander.

“At one point Kander had pulled a little bit ahead of Blunt,” said the source, who added that was the case two and a half weeks ago but claimed internal data now shows the race even.

Kander’s bump coincided with one of the best television ads of the cycle, which he aired to respond to attacks on his position on gun rights. It featured Kander, a veteran of the Army National Guard, assembling an assault rifle blindfolded in 30 seconds to prove he’s not a neophyte when it comes to firearms.

Democrats say they have internal polling that shows the race tied or Kander ahead and cite a trend across the surveys showing Blunt’s unfavorable rating on the rise since December.

Missouri was considered a second-tier race at the beginning of the cycle, but now it is among the handful of most competitive contests that will determine which party controls the Senate next year.

The Democratic Senatorial Campaign Committee in recent weeks has shifted funds previously earmarked for Ohio and Florida to Missouri, and outside third-party groups are pouring in millions of dollars.

Democrats are making the race a referendum on Blunt’s longtime membership atop the GOP establishment and the wealth his family has accumulated as a result.

Republicans are trying to paint Kander as a rubber stamp for Clinton if she wins the Oval Office.

“You have two competing themes here. One is Republican versus Democratic. The advantage there goes to Blunt because Missouri is a Republican-leaning state,” said the GOP strategist. “The other theme is insider versus outsider, where the advantage goes to Kander.”

The Senate Leadership Fund, an outside group linked to Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.), has committed to spending $7.8 million in Missouri in September and October, a big increase over the $2.5 million the group initially pledged to spend in September alone.

The National Rifle Association has so far spent $1.2 million in Missouri and sees the race as pivotal to keeping the Republican majority in the Senate and a check on any liberal judge that Clinton might nominate to the Supreme Court.

“Jason Kander has a long record of voting against the Second Amendment,” said Jennifer Baker, an NRA spokeswoman. “If elected, he would be a rubber stamp for an anti-gun Supreme Court justice who would vote to overturn law-abiding citizens’ fundamental right to have a firearm in the home for self protection.”

So far this cycle, Republican-allied outside groups have spent $9.7 million to protect Blunt, according to a Democratic source who tracks media buys.

Democratic-allied outside groups have spent $3.3 million in the state since Aug. 30, according to the source.

Between Aug. 30 and Oct. 10, Blunt, the National Republican Senatorial Committee and outside GOP groups have spent $8.15 million. Kander, the Democratic Senatorial Campaign Committee and outside Democratic groups have spent $7.5 million.

It’s one of the few competitive races in the country where Trump, who is relatively popular in Missouri, isn’t much of an issue.

“He is talking a little bit about what a Trump Washington would look like, but his main focus is to stay away from policy and ideological labels and instead focus on the characteristics of Blunt and the fact that he’s been in Washington so long and the lobbying connection,” said Steven S. Smith, a political science professor at Washington University in St. Louis.

Instead, it’s the Republicans who are trying to nationalize the race by tying Kander, a young Army vet who served in the state legislature before becoming state secretary, to Clinton.

An Emerson College poll from early September showed Clinton losing to Trump 34 percent to 47 percent.

Eyes in Missouri will be on the presidential race Sunday, when Clinton and Trump meet in St. Louis for their second debate.

The Senate Leadership Fund has aired three ads in Missouri hitting Kander for his record in the statehouse, targeting his votes in support of the Affordable Care Act, a national energy tax and government services for illegal immigrants. The group has also attacked him for serving as a national co-chair of Clinton’s outreach to millennials.

The Blunt campaign points out that Kander in 2009 signed a letter along with more than 1,000 other state legislators urging President Obama to pass comprehensive health reform legislation.

Kander is focused on Blunt’s long tenure in Washington and his wife’s and children’s careers as lobbyists.

“Sen. Blunt embodies Washington politics. He’s the only member of the Senate whose wife and whose all three children are lobbyists. He is tied into the lobbying community in a way that no other senator is,” said Chris Hayden, a spokesman for Kander’s campaign.

The DSCC’s first ad in Missouri highlighted Blunt’s 12 votes to increase congressional pay, his $7 million net worth, his $1.6 million Washington residence and his family's lobbying ties.

One Democratic operative pointed to a report published this week by Morning Consult revealing 17 times when Blunt’s position on a bill aligned with the position his wife had represented as a paid lobbyist.

The Missouri Republican Party on Thursday issued a press release criticizing Kander for missing nearly half of the meetings of the Missouri Veterans Commission in 2010 and 2011 while a state representative.

Hayden said his boss “was balancing a lot of different duties” and missed one meeting because he was participating in military drills as a member of the Missouri national guard.

The Kander campaign shared a research document showing that Blunt has missed a variety of national security-related votes in recent years, such as on a motion to extend provisions of the Patriot Act in 2011 and to advance the Department of Defense Authorization Act in 2013.

- This story was updated at 11:40 a.m.