Coons ran in the 2010 special election for the U.S. Senate seat then held by Democrat Ted Kaufman, who was appointed after Joe Biden resigned to take office as Vice President.
Kaufman had been appointed as a placeholder, and did not run in the special election.
Coons was unopposed in the Democratic primary, and expected to face Republican Congressman and former Governor Mike Castle in the general election.
He was considered a decided underdog in due to Castle's moderate profile and longstanding popularity in the state, but the dynamics of the race were significantly altered when Christine O'Donnell, a considerably more conservative Republican who had been Biden's opponent in 2008, upset Castle in the Republican primary.

In the first post-primary polls, Rasmussen Reports showed Coons with a double-digit lead over O'Donnell, describing this as a "remarkable turnaround" given that the race had leaned Republican before O'Donnell's primary victory.
In the first week of October, Fairleigh Dickinson University's PublicMind Poll showed Coons with a 17-point lead, 53%-36%, over O'Donnell, and that 85% of self-identified Democratic voters had united behind Coons, while only 68% of Republican voters endorsed O'Donnell.
Days before the election, a second Fairleigh Dickinson poll showed Coons leading 57% to 36% among likely voters, and 72% to 20% among voters who described themselves as moderates.
As polls closed at 8 p.m. Eastern Standard Time, multiple news sources announced that Coons had defeated O'Donnell based on exit poll data.
Final results gave Coons close to a 17-point margin over O'Donnell, with 56.6% of the vote to her 40%.

During the campaign, a controversy arose about an article Coons wrote in 1985 for his college newspaper titled "Chris Coons: The Making of a Bearded Marxist".
In it, he described his transformation from a Republican to what Fox News described as a "Democrat suspicious of America's power and ideals."
Dave Hoffman, a Coons campaign spokesman, said the title of the article was designed as a humorous take-off on a joke Coons's college friends had made about how his time outside the country had affected his outlook. "
After witnessing crushing poverty and the consequences of the Reagan Administration's 'constructive engagement' with the South African apartheid regime, he rethought his political views, returned to the America he loved and proudly registered as a Democrat," Hoffman said in a statement to Politico.

According to Fox News, Coons was "targeted by Republicans" over the 25-year-old piece.
Coons downplayed the article, as well as controversial past statements by O'Donnell, saying that voters were interested in current issues such as job creation and the national debt and not "particularly interested in statements that either of us made 20 or 30 years ago."
David Weigel wrote in Slate, "If the Tea Party Express slings the 'bearded Marxist' nonsense, I doubt it will work."
Coons ran in the 2010 special election for the U.S. Senate seat then held by Democrat Ted Kaufman, who was appointed after Joe Biden resigned to take office as Vice President. Kaufman had been appointed as a placeholder, and did not run in the special election. Coons was unopposed in the Democratic primary, and expected to face Republican Congressman and former Governor Mike Castle in the general election. He was considered a decided underdog in due to Castle's moderate profile and longstanding popularity in the state, but the dynamics of the race were significantly altered when Christine O'Donnell, a considerably more conservative Republican who had been Biden's opponent in 2008, upset Castle in the Republican primary.
In the first post-primary polls, Rasmussen Reports showed Coons with a double-digit lead over O'Donnell, describing this as a "remarkable turnaround" given that the race had leaned Republican before O'Donnell's primary victory. In the first week of October, Fairleigh Dickinson University's PublicMind Poll showed Coons with a 17-point lead, 53%-36%, over O'Donnell, and that 85% of self-identified Democratic voters had united behind Coons, while only 68% of Republican voters endorsed O'Donnell. Days before the election, a second Fairleigh Dickinson poll showed Coons leading 57% to 36% among likely voters, and 72% to 20% among voters who described themselves as moderates. As polls closed at 8 p.m. Eastern Standard Time, multiple news sources announced that Coons had defeated O'Donnell based on exit poll data. Final results gave Coons close to a 17-point margin over O'Donnell, with 56.6% of the vote to her 40%.
During the campaign, a controversy arose about an article Coons wrote in 1985 for his college newspaper titled "Chris Coons: The Making of a Bearded Marxist". In it, he described his transformation from a Republican to what Fox News described as a "Democrat suspicious of America's power and ideals." Dave Hoffman, a Coons campaign spokesman, said the title of the article was designed as a humorous take-off on a joke Coons's college friends had made about how his time outside the country had affected his outlook. "After witnessing crushing poverty and the consequences of the Reagan Administration's 'constructive engagement' with the South African apartheid regime, he rethought his political views, returned to the America he loved and proudly registered as a Democrat," Hoffman said in a statement to Politico.
According to Fox News, Coons was "targeted by Republicans" over the 25-year-old piece. Coons downplayed the article, as well as controversial past statements by O'Donnell, saying that voters were interested in current issues such as job creation and the national debt and not "particularly interested in statements that either of us made 20 or 30 years ago." David Weigel wrote in Slate, "If the Tea Party Express slings the 'bearded Marxist' nonsense, I doubt it will work."
Chris Coons