The Republican incumbent began to close in on Feingold in October. Feingold previously led by wide margins in multiple polls as Johnson faced an uphill race in an election overshadowed by his party's presidential nominee—Donald Trump. According to polling just a week before Election Day, Johnson lagged behind Feingold by just one point; TPM's PollTracker Average had Feingold leading by 0.6 points before the returns came in on Tuesday.
Johnson had unseated Feingold by a margin of 5 percentage points in the Democrat's own re-election race in 2010. He was considered one of the most vulnerable senators facing a re-election race in 2016, heading into the race with just a 42 percent approval rate, according to a Wisconsin Survey poll released in April.
Johnson has a rocky relationship with Trump, whose candidacy threatened to derail the senator's own race for re-election. He denounced Trump's comments about grabbing women by the genitals in a leaked "Access Hollywood" tape, saying said he would not "defend the indefensible."
In an October debate with Feingold, he refused to refer to Trump by name, instead calling him "our Republican nominee." Yet he agreed with Trump's positions on border security and fighting terrorism by the Islamic State in the same debate.
Less than a week afterward, Johnson broke with Trump's claims that the election would be "rigged" against him, and said that all candidates should "respect the results on Election Day."