WASHINGTON -- Groups trying to stop Chuck Hagel from becoming the next Defense Secretary have spent $123,000 on television ads against the Republican former senator from Nebraska, according to a new analysis by the Sunlight Foundation.

In the run-up to his confirmation hearings on Thursday, Hagel has faced a barrage of spending from anonymous donors hiding behind pop-up groups with names like "Use Your Mandate" and "Americans for a Strong Defense." Attack ads are common in political campaigns but rarely used to oppose Cabinet nominees.

Americans for a Strong Defense is run by former Mitt Romney campaign staffers, according to ABC News. The group has spent at least $63,000 on TV ads aimed at convincing Democratic senators up for reelection in 2014 to vote against Hagel.

One of the more secretive groups targeting Hagel is Use Your Mandate, which characterizes itself as being composed of mostly Democrats and independents who want to stay anonymous because they fear retribution from the White House. It has placed $47,000 worth of TV ads. The pop-up group has primarily focused on Hagel's weak record on LGBT issues.

Despite claiming it leans left, Use Your Mandate uses "Del Cielo Media, an arm of one of the most prominent Republican ad-buying firms in the country, Smart Media," The New York Times reported recently.

The American Future Fund, which spent $18.1 million against President Barack Obama in the 2012 election, spent $14,000 on an anti-Hagel ad that aired this week during Fox News Sunday. The ad questioned the former senator's personal financial assets, hoping to delay Hagel's hearing.

Other groups have placed ads online and in print. The Log Cabin Republicans, for example, has run anti-Hagel ads in both The New York Times and the Washington Post, and the group has refused to disclose the donors who have paid for them.

The secretive spending of these groups is part of the legacy of Citizens United, which loosened campaign finance rules and ushered in more anonymous political spending.

A national security group called the Bipartisan Group has come to Hagel's aid, according to the Sunlight Foundation, and placed approximately $35,000 worth of supportive ads online in Politico's Playbook.

Many of Hagel's opponents have been trying to portray him as weak in his support of Israel. That talking point, however, was undermined when Sen. Chuck Schumer (D-N.Y.) -- known for his staunchly "pro-Israel" record -- endorsed Hagel for Defense Secretary.

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  • Chuck Hagel (February 2013 - Present)

    New Secretary of Defense Chuck Hagel is greeted as he arrives for his first day at the Department of Defense, on February 27, 2013 in Arlington, Va. (Photo by Drew Angerer/Getty Images)

  • Leon Panetta (July 2011 - February 2013)

    Secretary of Defense Leon E. Panetta pauses while speaking during a ceremonial swearing-in at the Department of Defense July 22, 2011 in Washington. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Robert Gates (Dec. 2006 - July 2011)

    Robert Gates speaks during his ceremonial swearing in as the 22nd defense secretary on Dec. 18, 2006 at the Pentagon. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Donald Rumsfeld (Jan. 2001 - Dec. 2006)

    U.S. Secretary of Defense Donald Rumsfeld holds his press conference at the Pentagon briefing room on Jan. 26, 2001 in Arlington, Va. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • William Cohen (Jan. 1997 - Jan. 2001)

    Secretary of Defense designate William Cohen testifies during confirmation hearings before the Senate Armed Services Committee on Jan. 22, 1997 in Washington. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • William Perry (Feb. 1994 - Jan. 1997)

    U.S. Defense Secretary William Perry points to a reporter during a press conference on April 21, 1994 in Seoul, Korea. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Les Aspin (Jan. 1993 - Feb. 1994)

    U.S. Secretary of Defense Les Aspin released new regulations governing gays in the military during a press on Dec. 22, 1993 at the Pentagon. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Dick Cheney (March 1989 - Jan. 1993)

    U.S. Defense Secretary Dick Cheney (L) meets Egypt's President Hosni Mubarak, on April 3, 1989, at Washington. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Frank Carlucci (Nov. 1987 - Jan. 1989)

    U.S. Defense Secretary Frank Carlucci testifies before the Senate Armed Services Committee on July 13, 1988 in Washington. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Caspar Weinberger (Jan. 1981 - Nov. 1987)

    Caspar Weinberger, Secretary of Defense on Feb. 9, 1981. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Harold Brown (Jan. 1977 - Jan. 1981)

    General Alexander M. Haig, right, retired as NATO commander, walks with Defense Secretary Harold Brown during an awards ceremony on July 3, 1979 at Fort Myer, Va. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Donald Rumsfeld (Nov. 1975 - Jan. 1977)

    A 1976 photo of Secretary of Defense Donald Rumsfeld. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • James Schlesinger (July 1973 - Nov. 1975)

    Israeli Defense Minister Moshe Dayan, left, with Secretary of Defense James R. Schlesinger, chats on Friday, Jan. 5, 1974 at the Pentagon. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Elliot Richardson (Jan. 1973 - May 1973)

    Elliot L. Richardson speaks to newsmen Oct. 23, 1973 at a press conference held at the Department of Justice. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Melvin Laird (Jan. 1969 - Jan. 1973)

    Secretary of Defense Melvin R. Laird as he departed from Andrews Air Force Base Md., for Paris on Jan. 5, 1971 in Washington. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Clark Clifford (March 1968 - Jan. 1969)

    This is an Oct. 1968 photo of Secretary of Defense Clark Clifford as he announces his support for President Johnson's decision to halt the bombing of North Vietnam. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Robert McNamara (Jan. 1961 - Feb. 1968)

    PARIS, FRANCE: US Secretary of Defense Robert McNamara smiles as he arrives 27 November 1965 at Paris' NATO headquarters. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Thomas Gates (Dec. 1959 - Jan. 1961)

    Secretary of Defense Thomas S. Gates Jr., center, poses with Benjamin M. McKelway, left, editor of the Washington Evening Star and President of the AP, and AP General Manager Frank J. Starzel at the April 25, 1960 meeting of the Associated Press in New York. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Neil McElroy (Oct. 1957 - Dec. 1959)

    Defense Secretary Neil McElroy said he has "fullest confidence that the United States is ahead of the Soviets..." prior to the announcement of the Soviet's achievement in launching the first earth satellite, Oct. 4, 1958. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Charles Wilson (Jan. 1953 - Oct. 1957)

    Charles E. Wilson, left, takes the oath of office from Chief Justice Fred Vinson at the White House in Washington on Dec. 21, 1950 as head of the office of Defense Mobilization. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Robert Lovett (Sept. 1951 - Jan. 1953)

    Gen. Dwight D. Eisenhower (right) watches President Harry S. Truman and Gen. Omar Bradley help Defense Secretary Robert Lovett (left) get in place as the men posed on the south lawn of the White House on June 1, 1952 in Washington. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • George Marshall (Sept. 1950 - Sept. 1951)

    Anna M. Rosenberg, New York Labor and Public Relations consultant, who is named assistant secretary of defense, chats with Secretary of Defense George Marshall in a conference at the Pentagon on Nov. 9, 1950 in Arlington, Va. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • Louis Johnson (March 1949 - Sept. 1950)

    Defense Secretary Louis Johnson (right) gives new identification card to President Harry Truman at the White House on Nov. 9, 1949 in Washington, listing him as commander in chief for an "indefinite" term. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)

  • James Forrestal (Sept. 1947 - March 1949)

    James V. Forrestal, Secretary of Defense under President Harry Truman, is shown on July 26, 1947. (Source: <a href="http://www.defense.gov/specials/secdef_histories/">Department of Defense</a>)