Iowa will host one of the most closely watched U.S. Senate races this election cycle.

This is surprising because it involves Republican Senator Charles Grassley, Chairman of the Senate Judiciary Committee. Referred to in Iowa as “the 800-pound gorilla” for electoral invincibility in a political career spanning 56 years, he has received no less than 64% of the vote in each of his five previous re-election campaigns to the U.S. Senate.

Two recent polls, however, show his support at just 47% and holding only a single-digit lead over his Democratic challenger.

While Senator Grassley’s defeat might seem unimaginable, Iowa has a recent history of giving forced retirements to long serving and distinguished Federal legislators. Democratic Congressman Neal Smith lost re-election in 1994 after serving 36 years in the U.S. House of Representatives, coincidentally, the same number of years Grassley has served in the U.S. Senate. Republican Congressman Jim Leach lost re-election in 2006 after serving 30 years in the House. Like Grassley, both had commandingly won their previous elections with 60% of the vote, but lost their final races in historic election years in which control of the House of Representatives switched parties.

Grassley, first elected to the U.S. Senate in 1980 when Barack Obama was a 19-year-old college sophomore at Occidental College in Los Angeles, would be cruising to a seventh term if it wasn’t for one major decision: his refusal to hold hearings for Judge Merrick Garland, President Obama’s nominee to the Supreme Court.

His decision drew bipartisan criticism in Iowa. Iowa’s largest paper, The Des Moines Register, in an editorial on April 4, called Grassley’s refusal to hold hearings, “pure partisanship – and simple stubbornness.” They called his decision, “unsatisfying. And un-American.”

His decision also drew a late Democratic challenger into the race, a Judge who brings the most impressive resume of any of Grassley’s previous opponents. No, not a black-robed lawyer rendering verdicts in a court of law, but Patty Judge, a farmer from Alba who is a former Iowa Lieutenant Governor and Secretary of Agriculture. The irony of the name is not lost on anyone, particularly Judge, whose campaign tagline is “I’m the Judge Grassley can’t ignore.”

Grassley’s refusal to hold hearings is seen by many in Iowa as a disappointing development for a senator who has carefully sculpted an image as an independent and bipartisan legislator. In visiting each of Iowa’s 99 counties every year, his famous tours of Iowa known as “the full Grassley,” he has cultivated this image and presented himself a trusted farmer-next-door legislator. This has allowed him to successfully localize previous elections and earn the votes of tens of thousands of democrats and dominate among independents.

It’s hard to imagine Iowa Democrats continuing to support Grassley in this election. Iowa Democrats have joined with Independents in delivering two victories a piece to Bill Clinton and Barack Obama and a string of five election victories to liberal U.S. Senator Tom Harkin before his retirement in 2015. With Hillary Clinton at the top of the ticket in this presidential year, it’s difficult to believe that Iowa Democrats would split their ballot and support unprecedented obstructionism that Grassley has chosen to participate in.

While Grassley has dominated independent voters in the past, whether they will support him in this election remains a big question.  According to information available on the Iowa Secretary of State’s website, since Grassley’s last election in 2010 registered voters listing “No Party” has grown by a staggering 100,000 voters. At 35% of total registered voters in Iowa, this is now the largest block of voters in Iowa. Whether these voters, ostensibly choosing not to identify with a party because of partisan politics, will vote for a party leader steadfastly committed to divisive partisan politics in Washington remains to be seen.

These independent voters will also have to consider the reality of Grassley’s support for Donald Trump. Unlike his Senate colleagues up for re-election running from Trump and ducking for cover, Grassley has drawn close to Trump. He introduced Trump in Iowa at a campaign rally in January and officially declared his support in May when Trump became the presumptive Republican presidential nominee. In June, Grassley said Trump would nominate the right type of person to the Supreme Court and, as reported in The Hill, when asked if Grassley would campaign with Trump he replied, “The answer is yes … If he asks me to campaign with him, I would.”

With Donald Trump at the top of the Republican ticket, many Republicans fear a catastrophe for down ballot races. If an anti-Republican wave materializes, an “if” given the unpredictability of this election season, Smith and Leach’s losses prove that Iowa’s borders are not impenetrable to forces sweeping the country.  Long incumbencies, distinguished careers, and previous dominating election successes offer no protection.

Whether or not Iowa voters send the 800-pound gorilla back to Washington will determine control of the U.S. Senate. If they don’t, it won’t be because Iowa voters abandoned Chuck Grassley. It will be because Chuck Grassley abandoned Iowa voters and the very principles of bipartisanship and independence that allowed him to dominate Iowa politics for nearly half a century.