On Thursday, former Sen. Chuck Hagel will appear before the Senate for his confirmation hearings as secretary of defense, but unlike recently appointed Secretary of State John Kerry, who breezed through the process, he faces a tough time. Hagel’s high-profile critics, big-money lobbyists, and political pundits are taking up arms against the man they see as a threat to the U.S. military. Why are they spending hundreds of thousands of dollars to ensure he doesn’t get confirmed?

The vitriol was unleashed when Hagel’s name was floated in early December as an Obama favorite for the defense portfolio, and ramped up upon his nomination in early January. Republicans didn’t quite see the former two-term senator—who co-sponsored a resultion against the troop surge in Iraq, opposes unilateral Iranian sanctions, and supports a scaled-back military—as one of their own. “[To] allege that Hagel is somehow a Republican—that is a hard one to swallow,” Sen. John McCain, the top Republican on the Armed Services Committee, told Politico, in the first of a string of attacks that would soon hit the airwaves and the Internet.

The anti-Hagel campaign goes past talk-show appearances and op-ed pieces. Hagel’s harshest opponents make a strange group—a mix of conservative Republicans, LGBT rights groups, and pro-Israel lobbyists. And they’re spending big to keep him out of the secretary of defense job. On Tuesday, an analysis by the Sunlight Foundation found that $123,000 has been spent on anti-Hagel TV ads by these outside groups. US News reported that at least five of these groups have formed an anti-Hagel campaign, buying up TV, newspaper, and online ads in an attempt to influence senators against voting for him. Even the domain chuckhagel.com was snapped up by the nonprofit Emergency Committee for Israel, and comes up as an ad at the top of Google search results with a homepage that screams, “Chuck Hagel is Not a Responsible Option.”

Hagel’s positions and off-the-cuff comments haven’t endeared him to the pro-Israel lobbyists and politicians. He once declared that he’s “not an Israeli senator,” threw out a remark about the “Jewish lobby,” and criticized the country’s dealings with Lebanese militant group Hizbullah.

“Chuck Hagel’s record on Israel, on Iran, and on other vital defense issues is cause for real concern,” Republican Jewish Coalition Executive Director Matt Brooks said in a press release for the release of an ad urging senators not to vote for Hagel.

While many Democrats seem to back Obama’s pick, the sentiment against Hagel isn’t entirely Republican. Rep. Eliot Engel, the top Democrat on the House Foreign Affairs Committee, shared concerns over Hagel’s Israel stance in an interview on C-SPAN, saying, “It seems there is some kind of an endemic hostility towards Israel and that’s troublesome to me and troublesome to a lot of people.” Not all agree—in late December, nine former U.S. ambassadors, five who served in Israel, wrote a letter of support for Hagel.

Despite Hagel’s decorated military service, Republicans have expressed skepticism about his past defense stances—the same ones Obama likely picked him for. On Jan. 25, Hagel was dealt another blow when Sen. Jim Inhofe, the ranking Republican on the Senate Armed Service Committee, wrote an op-ed in The Washington Post, asserting that he is “unable to support [Hagel’s] nomination,” due to Hagel’s agreement with Obama that the Pentagon budget should be scaled back. Inhofe also railed against the nominee for supporting moderate nuclear disarmament, writing that the U.S. can’t take that risk as Iran and North Korea try to build their arsenals.

Gay-rights groups on both political sides have joined the fray, bashing Hagel for an incident in 1998, when he called Ambassador James Hormel “openly, aggressively gay.” (Hormel has since accepted Hagel’s apology). Opponents formed Use Your Mandate, an LGBT advocacy group launched to put pressure on Hagel, which has spent tens of thousands on a TV ad, anti-Hagel mail, and a banner ad on Politico.

Other LGBT organizations have criticized the tactics of Use Your Mandate, and praised Hagel for apologizing, and “taking steps to build trust with our LGBT military families.”

In the end, it boils down to the Senate. “I can tell you there would be very little Republican support for his nomination. At the end of the day, there will be very few votes,” Republican Sen. Lindsey Graham told FOX News Sunday before Hagel’s nomination was officially announced. At the same time, Senate Armed Services Chairman Sen. Carl Levin told Politico that if Hagel is nominated, “he’ll be fine.”

We’ll see what happens Thursday.

Editors' Note: The original version of this piece stated that Chuck Hagel "voted against the Iraq war." The story has been changed to reflect that he co-sponsored a resolution against the troop surge in Iraq.