Question: Who was the Republican challenger when former Secretary of State Hillary Clinton was elected to the senate in 2006?

Context paragraphs:
Peter Mandaville: Peter Mandaville is an American academic and government official.  Since 2015 he has been Senior Advisor in the Secretary of State's Office of Religion & Global Affairs at the U.S. Department of State.  His previous government work has included serving as a member of the State Department's Policy Planning Staff (2011-12) under former Secretary of State Hillary Clinton, where he was part of the team that helped to shape the U.S. response to the Arab Spring.

United States Senate election in New York, 2006: The 2006 United States Senate election in New York was held November 7, 2006.  Incumbent Democratic U.S. Senator Hillary Rodham Clinton won, by a more than two-to-one margin, a second term representing New York in the United States Senate.  Clinton was challenged by Republican John Spencer, a former Mayor of Yonkers, New York.  Spencer won his party nomination by defeating former national security staffer K. T. McFarland in a primary.

Dennis Ross: Dennis B. Ross (born November 26, 1948) is an American diplomat and author.  He has served as the Director of Policy Planning in the State Department under President George H. W. Bush, the special Middle East coordinator under President Bill Clinton, and was a special adviser for the Persian Gulf and Southwest Asia (which includes Iran) to the former Secretary of State Hillary Clinton.

DipNote: DipNote is the official blog of the United States Department of State.  It was started on September 25, 2007 with a first post by the U.S. Assistant Secretary of State for Public Affairs Sean McCormack.  DipNote was revised in 2009, with the Departments' 21st Century Statecraft initiative.  DipNote works in conjunction with other public diplomacy programs as part of former Secretary of State Hillary Clinton's smart power initiatives.

United States Senate career of Hillary Clinton: Hillary Rodham Clinton served as a United States Senator from New York from January 3, 2001 to January 21, 2009.  She won the United States Senate election in New York, 2000 and the United States Senate election in New York, 2006.  Clinton resigned from the Senate on January 21, 2009 to become United States Secretary of State for the Obama Administration.

United States presidential election in Iowa, 2016: The 2016 United States presidential election in Iowa was held on November 8, 2016, as part of the 2016 General Election in which all 50 states plus The District of Columbia participated.  Iowa voters chose electors to represent them in the Electoral College via a popular vote pitting the Republican Party's nominee, businessman Donald Trump, and running mate Indiana Governor Mike Pence against Democratic Party nominee, former Secretary of State Hillary Clinton and her running mate, Virginia Senator Tim Kaine.

Indira A.R. Lakshmanan: Indira A.R. Lakshmanan is a Washington DC-based columnist for the Boston Globe, writing about foreign policy and politics, and the Newmark Chair for Journalism Ethics at the Poynter Institute.  She is a sometime panelist on Washington Week on PBS.  Her Washington Week profile notes that "She has covered presidential campaigns and interviewed leaders in the U.S. and around the world, reporting from 80 countries on six continents.  She has traveled with the campaigns of Barack Obama, Hillary Clinton, John McCain, and Mitt Romney and traveled regularly with Secretaries of State Hillary Clinton and John Kerry for Bloomberg News, and interviewed Clinton more than a dozen times for Bloomberg TV, Radio and Businessweek."

United States presidential election in Ohio, 2016: The 2016 United States presidential election in Ohio was held on November 8, 2016, as part of the 2016 General Election in which all 50 states plus The District of Columbia participated.  Ohio voters chose electors to represent them in the Electoral College via a popular vote pitting the Republican Party's nominee, businessman Donald Trump, and running mate Indiana Governor Mike Pence against Democratic Party nominee, former Secretary of State Hillary Clinton and her running mate, Virginia Senator Tim Kaine.

United States presidential election, 2016: The United States presidential election of 2016 was the 58th quadrennial American presidential election, held on Tuesday, November 8, 2016.  In a surprise victory, the Republican ticket of businessman Donald Trump and Indiana Governor Mike Pence defeated the Democratic ticket of former First Lady, U.S. Senator of New York and Secretary of State Hillary Clinton and U.S. Senator from Virginia Tim Kaine.  Trump took office as the 45th President, and Pence as the 48th Vice President, on January 20, 2017.  Concurrent with the presidential election, Senate, House, and many gubernatorial and state and local elections were also held on November 8.

United States presidential election in Utah, 2016: The 2016 United States presidential election in Utah was held on November 8, 2016, as part of the 2016 General Election in which all 50 states plus The District of Columbia participated.  Utah voters chose electors to represent them in the Electoral College via a popular vote pitting the Republican Party's nominee, businessman Donald Trump, and running mate Indiana Governor Mike Pence against Democratic Party nominee, former Secretary of State Hillary Clinton and her running mate, Virginia Senator Tim Kaine.

Supporting paragraphs: United States Senate career of Hillary Clinton United States Senate election in New York, 2006
Answer: John Spencer