In 2010, then-incumbent Democratic senator
Russ Feingold ran for re-election to a fourth term
in 2010 and was defeated by Republican nominee
Ron Johnson. In March 2013, Johnson announced that he had begun fundraising for his campaign. At that time, he had just $1,529 remaining in his campaign account after raising $16.1 million for the 2010 election, over half of which he self-funded. Johnson said in November 2014 that he would not self-finance another campaign, saying: "I made my $9 million investment in this country. I gave it once, I don't think I should do it again." On May 14, 2015, Feingold announced he would run to win back his former Senate seat. Ultimately, Feingold spent over $24 million on the campaign and ended up with more remaining cash than Johnson, who spent only $20 million. After the Republicans took control of the Senate following the
2014 Senate elections, the election in Wisconsin was seen by many as a top target for the Democrats, who hoped to retake their majority in the traditionally blue state.
Politico pointed to Johnson's "worrisome" favorability ratings as one of the main reasons for his vulnerability. == Republican primary ==