Senate leaders began hunkering down Wednesday for an expected tense showdown over new Federal Election Commission nominations that came down earlier this week from President Bush.

“The devil’s in the details,” said Jim Manley, a spokesman for Senate Majority Leader Harry Reid (D-Nev.), of upcoming negotiations with the White House and Senate Republicans.

But Senate Minority Leader Mitch McConnell (R-Ky.) set out early markers on the new deal Wednesday, suggesting that he will stand by his demand that all nominees be considered at once.

“The Democrats will not be able to … determine who the Republican nominees are, any more than we want to determine who theirs are,” McConnell said Wednesday. “So they’ll either all six go in rough proximity to each other or we will not have solved the problem.”

In an attempt to bring the FEC back online, the White House on Tuesday nominated Cynthia Bauerly, Don McGahn and Caroline Hunter to three spots on the six-member elections panel. Along with these nominations, Bush said he intended to pursue two holdover nominations, Democrat Stephen Walther and Republican Hans von Spakovsky.

von Spakovsky’s nomination has been particularly toxic to Democrats and civil rights groups, stemming from his tenure at the Justice Department. The commission has essentially been out of business since the beginning of the year because a standoff over the FEC nominations has left the panel without a quorum.

Bauerly, an aide to Democratic Senatorial Campaign Committee Chairman Charles Schumer (N.Y.), would replace former chairman Robert Lenhard, who stepped down from consideration last month to take a job at Covington & Burling LLP.

Caroline Hunter, a Republican pick who currently sits on the Election Assistance Commission, would take over the seat held previously by Michael Toner, now a lawyer at Bryan Cave.

Democratic commissioner Ellen Weintraub, whose term has expired, would remain on the commission as part of the White House’s proposal.

But the White House’s decision to replace GOP chairman David Mason with McGahn, a one-time lawyer to former House Majority Leader Tom DeLay (R-Texas) who’s now in private practice, is threatening to dismantle the delicate political matrix.

By Roll Call press time Wednesday, campaign finance reform groups were distributing a laundry list of McGahn’s alleged ideological sins.

“In nominating Don McGahn to replace David Mason on the FEC, the White House and Senate Republican Leader Mitch McConnell have chosen an individual whose past record shows disdain for the FEC and little interest in the proper enforcement of campaign finance laws,” Democracy 21 President Fred Wertheimer said in a statement.

Wertheimer and other reformers were also in an uproar on the White House’s decision to make room for McGahn by firing Mason, the current Republican-nominated chairman.

Wertheimer called Mason’s ouster “political obstruction of justice.”

Twice nominated by Bush, Mason sat on the commission for nearly a decade. But he frequently riled conservatives for what they deemed an apolitical, pragmatic style.

“Dave had an approach … which is that the best way to run the agency [by trying] to reach some agreement among the commissioners,” said Lenhard, Mason’s former colleague. “This is an agency where as commissioner you don’t make a lot of friends [and] the longer your are there, the fewer friends you make.”

A Republican campaign finance insider agreed with Lenhard’s suggestions that conservative lawmakers had grown frustrated with Mason’s unpredictability — albeit mostly the idle water cooler variety.

But GOP grumbles grew into outrage in recent months, the source said, particularly when Mason bucked his party’s priorities in February by disputing Sen. John McCain’s (R-Ariz.) claims that he could opt out of the presidential public financing system.

“A lot of people are, like, what the heck is going on?” a GOP election law practitioner said. “There’s been a certain level of dissatifcation with Commissioner Mason among certain Republicans for quite a long time.

“There’s a sense that he’d become captive to the agency, and a lot of people have been disturbed about what the agency has been doing for the last four months,” the source said. “You’ll find among Republican in Congress and Republican election lawyers that there was perhaps” a better nominee.

How Senate negotiators structure the vote on von Spakovsky and Walther’s nominations could threaten to topple the political house of cards.

McConnell and other Senate Republicans for months have taken an all-or-nothing approach to Republican FEC picks — particularly with von Spakovsky — demanding that agency nominees be voted on as a group, a position he reiterated Wednesday.

“We have picked three that we’re comfortable with,” McConnell said. “The president’s nominated three that they’re comfortable with. Now Sen. Reid and I need to work out a process by which we can get all six of them confirmed.”

The White House is hoping that a recent Supreme Court decision may be providing some wiggle room for both sides on von Spakovsky, who Republicans say was recently exonerated by the high court when it upheld Indiana’s voter identification law.

“This ruling vindicated his good faith legal position,” White House spokeswoman Emily Lawrimore wrote in an e-mail Wednesday. “And as a member of the Commission, he has clearly demonstrated his commitment to ensuring fair elections in our country.”

Lawrimore also said the president would like the Senate to vote on the nominations by Memorial Day.

Should Senate leaders agree to hold separate votes on the five nominees, all eyes are expected to be on Sen. Ben Nelson (D-Neb.), likely the tie-breaking vote should a full Senate vote on von Spakovsky split down party lines. Nelson’s office confirmed Wednesday that the conservative lawmaker is still “undecided.”

“Offers are going back and forth between leadership and the White House on the FEC nominees. I’m waiting to see what, if anything, they get worked out,” Nelson said in a statement. “We should let the votes fall where they may.”