Now that the GOP presidential field has winnowed down to one candidate with Rick Santorum’s exit this week, Karl Rove is free to turn his complete attention—and his super PAC’s checkbook—towards defeating President Barack Obama.

(That is not to say that the former senior advisor to George W. Bush ever viewed Santorum as a serious threat to Romney’s prospects: Rove has taken to the op-ed pages of the Wall Street Journal twice this month, first to declare Rick Santorum’s campaign all but dead and then to celebrate its demise. “Rick Santorum’s decision Tuesday to suspend his campaign effectively ends the GOP nomination fight. But it doesn’t mark the start of the general election between Mitt Romney and Barack Obama. That contest has long been under way,” Rove wrote in a piece published Wednesday.)

A $1.7 million ad buy in six swing states by Rove’s super PAC American Crossroads gave political junkies an early sense of how “Bush’s Brain” will escalate things now that the race is down to two candidates. The ad, titled “Too Much,” blames the Obama Administration’s energy policies for high gas prices and credits any rise in drilling to the Bush Administration. “No matter how Obama spins it, gas costs too much,” the narrator says in the 31-second ad, which will air in Virginia, Florida, Colorado, Iowa, Ohio, and Nevada.

And there’s plenty more cash where that came from: the Washington Post‘s Chris Cillizza and Aaron Blake point out that American Crossroads had $23.6 million in its war chest at the end of February, almost $21 million more than Priorities USA Action, a super PAC staffed by former Obama aides. What are the implications of this?

What Crossroads’ fundraising heft means — particularly when coupled with the lack of a similar success on the Democratic side as of yet — is that the group is positioned to do the dirty work of pulling Obama’s numbers in swing states down over the next two to three months, while Romney works to rehab his image and move to the ideological center.

In his Wednesday Wall Street Journal op-ed, Rove predicts Obama will take the “low road” in the general election and offers Romney some bits of advice:

Mr. Romney will need to tap into the disappointment and regret that many Americans, even the president’s supporters, feel about Mr. Obama. Yet while setting the record straight about the last three dismal years and Mr. Obama’s attacks is important, it is not enough. Winning candidates for the American presidency offer a positive, optimistic agenda that reassures voters about what they will do once in the White House.

Mr. Romney also should remind Americans of Mr. Obama’s lofty words from his 2008 acceptance speech at the Democratic Convention in Denver. There he said, “If you don’t have any fresh ideas, then you use stale tactics to scare voters. If you don’t have a record to run on, then you paint your opponent as someone people should run from.”

Mr. Obama attacked such a strategy then. Lacking any fresh ideas or a record to run on, it’s the strategy he’s adopted now.