Eric Holder’s decision to resign is the clearest indication yet that the White House believes there is a real possibility that Democrats will lose control of Congress in the coming midterm elections. If the GOP were to take control of the Senate with Holder in place as AG, he would likely have to serve in that role for the remainder of the Obama presidency.

Holder has, however, said that he intends to remain in his post until his replacement is confirmed. Even if Republicans take control of the Senate after the November elections, there will be a push to confirm Obama’s new nominee in the lame duck session. A confirmation fight over the supremely important post of attorney general looms, and it will probably not be a fight tackled by the 114th Congress’s Senate. Nevertheless, the politics of a confirmation fight is likely aimed squarely at the midterm electorate. Republicans and Democrats both stand to benefit from this shot in the arm.

Holder is the fourth-longes-serving attorney general in the nation’s history. His resignation was not unexpected, given that attorneys general do not usually serve out two full terms. Conventional wisdom holds that he is only stepping down now in order to provide unenthusiastic liberal Democrats with a reason to rally around the Democratic majority in the Senate. Obama can accomplish this by nominating a liberal who would have a tough time being confirmed by a GOP-dominated upper chamber as Holder’s replacement.

The two leading candidates who fit this bill are California Attorney General Kamala Harris and Massachusetts Gov. Deval Patrick.

“Atty. Gen. Kamala Harris is a veteran prosecutor, a skilled political leader and a promising politician who is sure to figure in California’s future,” read the Los Angeles Times endorsement of Harris for a second term. “Harris is sometimes accused of excessive partisanship — some say she crafts ballot language on initiatives that favor her side of an argument — and there is no denying her ambition; a run for governor seems likely. But there’s nothing wrong with ambition so long as it’s constructively channeled, and Harris also has demonstrated that she is capable of putting her professional responsibility over her politics.”

Ambition: Check. Partisan liberal: Check. Demographic desirability (Harris is both female and African-American): Check.

Patrick, meanwhile, is a known quantity and, having occupied statewide office and served as a surrogate for Barack Obama, his nomination may even be seen as more partisan and divisive than would Harris’s. Patrick has also served as a Department of Justice official in the past as the Assistant Attorney General for Civil Rights, so he has the qualifications and possibly even the desire to serve as Obama’s AG.

“Deval Patrick has always been a name you heard a lot over the years,” Meet the Press moderator Chuck Todd opined on Thursday. “He’s somebody that has been close to a lot of the president’s political aides.

“As somebody whose term is expiring in Massachusetts, electing a successor there, he expressed interest in – I think he would love to be a judge someday,” Todd continued. “I think he said one time in an interview one of his dream jobs would be to be a Supreme Court justice.”

But while base liberals are likely to be fired up over either nominee, conservatives will be just as excited over Holder’s resignation and the opportunity to replace him with someone they believe will not function as an overt political operative. That means ensuring that the lame duck Democratic Congress conducts itself with the understanding that they have been robbed of a mandate by the electorate in November.

The Senate could hang on the race in Kansas where Sen. Pat Roberts (R-KS) has had a near impossible time of regaining the trust of his base voters. Even former Alaska Gov. Sarah Palin has parachuted in to campaign with Roberts and shower him in conservative credentials. Even if that strategy fails to regain the trust of conservatives, the prospect of regaining the Senate and sending a message to the soon-to-be minority Democrats who will control the confirmation process may lead conservatives to hold their nose at the polls.

If Roberts retains his seat, the current polling environment suggests a GOP takeover of the upper chamber is very likely. Liberal enthusiasm may prevent Democrats from losing more than six seats, but the GOP will win retake the majority in the Senate even if they only pick up seats in states Mitt Romney won handily in 2012.

Of course, all this conventional wisdom may be thrown out if the White House wants to avoid a divisive confirmation battle and nominates a career prosecutor with a low profile. It is, however, almost October in an election year. The laws of political physics suggest that the White House will take full advantage of the opportunity presented by Holder’s resignation.