"There was a time when Republicans liked Jack Lew," President Obama's chief of staff and nominee to replace Timothy Geithner as Treasury secretary, says Ezra Klein at The Washington Post. "That time is no longer." Senate Republicans, already gearing up for a fight over Obama's nomination of former Sen. Chuck Hagel (R-Neb.) for defense secretary, are now promising a battle over Lew, also: Sen. Jeff Sessions (Ala.), the top Republican on the Senate Banking Committee, is even threatening to filibuster the nomination, vowing in a press release, "Jack Lew must never be Secretary of Treasury." What do Republicans have against the quiet, slightly nerdy Orthodox Jew who brings his own lunch — a cheese sandwich and apple — to work at his West Wing desk and still resides in the Bronx? Here, five reasons behind the GOP's newfound hatred of Jack Lew:

1. He's a tough, wonkish negotiator
Lew has worked in Washington since 1973, and until 2011 he had a reputation as "a committed progressive who Republicans liked and thought they could do business with because he's also a pretty hard-boiled numbers guy," says Matthew Yglesias at Slate. Then came the budget wars of 2011, when Lew — then director of the White House Office of Management and Budget — used his deep knowledge of budgetary math against them, refusing to let Republicans offset spending cuts with vague tax-reform "flim-flam." Don't let Lew's "nerdy exterior" fool you, says Massimo Calabresi at TIME. He's a fierce negotiator and "a passionate progressive on the issue of wealth disparity and programs for the poor" — Democrats credit him with saving Medicaid from GOP scalpels. Republicans simply don't want to face such "a formidable opponent as Washington heads into more tough negotiations over the budget."

2. Republicans feel he's condescending and wily
It's not just that Lew is a "wonk Zelig" — he has "played a role in every big budget deal since 1983," says Joshua Green at Bloomberg Businessweek. Republicans chafed at "his insistence on explaining in detail the impact of the cuts Republicans were proposing" during the 2011 negotiations. In fact, "Republicans found Lew condescending, inflexible, and a bit sneaky," says The Washington Post's Klein. Democrats dismiss these complaints as "sour grapes," but as the GOP sees it, Lew "often seemed to be trying to trick them into agreeing to complicated policies that ultimately redounded to the administration's benefits." And in at least one case, he did outfox the GOP.

3. Lew is a Democrat's Democrat
Geithner, the outgoing Treasury secretary, is also wonkish, but he's mostly a financial technocrat. Lew is a partisan Democrat, and his résumé is impressively full of liberal lore: During his one year at Minnesota's Carleton College, Lew's academic adviser was Paul Wellstone, who would go on to become a progressive icon in the U.S. Senate; Wellstone convinced him to enter politics, and his first big job in Washington was budget analyst and negotiator for legendary House Speaker Tip O'Neill, alongside MSNBC star Chris Matthews; and he was Bill Clinton's OMB director and later Hillary Clinton's deputy secretary of state before joining the Obama White House. Given the extreme polarization in Washington, this history doesn't exactly endear him to Republicans.

4. He agrees with Obama
"When it comes to worldview," says Noam Scheiber at The New Republic, "Lew is Obama in coke-bottle glasses." That actually makes some Democrats nervous, given both men's "romantic [views] about the virtues of a deficit grand-bargain." But it makes even more Republicans livid, given their hatred of Obama, says Jonathan Chait at New York. It's absurd, but "Lew's disqualification," according to Sessions and other Republicans, seems to boil down to "that he doesn't agree with Republicans on public policy issues." This is Obama's Cabinet, and it should reflect his policy views, of course. But I guess in today's Washington "Lew is unacceptable because Republicans want to pick the person on both sides of the negotiating table."

5. Republicans are just jockeying for leverage
Among the real sense of being burned by Lew is at least an element of crass political theatrics. The looming confirmation fight is a rare opportunity to grill the president's chief of staff, and "Republicans say Jack Lew will have to answer for what they view as the president's bare-knuckle tactics," says Alexander Bolton at The Hill. The GOP is especially "flummoxed... by Obama's blunt refusal to even negotiate legislation to raise the debt ceiling," unlike in 2011, when Bill Daily was chief of staff, and they blame Lew for the change in tactics. Republicans might also use Lew's confirmation hearings as leverage to extract steep spending cuts in the upcoming budget fight. "They would be foolish not to use this opportunity to lay the foundation for their strategy on the debt ceiling," political scientist Larry Sabato tells The Washington Examiner. This is one "high-profile hearing that they don't want to let go to waste."