Associated Press

Obama in Jacksonville for a Thursday rally.

President Obama began a two-day campaign swing through Florida yesterday -- he will, however cut short his stay, canceling an event in Winter Park in light of the tragedy in Aurora -- stressing an issue that has largely been in the background of late: Medicare.

President Obama opened a two-day campaign swing through Florida on Thursday as he tried to build support in this deadlocked battleground state by presenting his opponent, Mitt Romney, as a bad choice for older Americans.

After weeks of focusing on Mr. Romney's private-sector business deals, Mr. Obama turned to another front by attacking Republican plans to repeal his health care law and transform Medicare into a voucher program. Democrats have long used Medicare as an issue to galvanize older voters in Florida against Republicans.

"He plans to turn Medicare into a voucher program," Mr. Obama told supporters at a retirement community [in West Palm Beach]. "If the voucher isn't worth what it takes to buy health insurance in the private marketplace, you're out of luck, you've got to make up the difference, you're on your own."

This is a potent message for a few reasons. First, it's true. Second, it resonates with nearly everyone, especially retirees in the Sunshine State, since Medicare is so wildly popular.

And third, it reinforces a larger message that's critically important to the president's election strategy. Note, for example, this line from yesterday's remarks: "It's wrong to ask seniors to pay more for Medicare just so millionaires and billionaires can pay less in taxes."

As Jon Chait noted, "The Obama campaign's attacks on Mitt Romney's business record and personal finances will probably continue for a long time. But I think that, when the campaign is remembered in history, they will not be seen as the central element but rather as a prelude. The main event is going to be a fight over the priorities of the Paul Ryan budget."

There may be some who characterize this as Democrats giving up on the focus on Bain and hidden tax returns. But that's almost certainly wrong.

The larger effort is intended to be cumulative, building one argument on top of another. Phase I was intended to discredit Romney's controversial past; Phase II appears intended to discredit Romney's plans for the future.

And when it comes to Medicare, the idea of replacing the popular and effective system with a voucher plan -- all so Republicans can help pay for more tax cuts for the rich -- is an argument Obama is eager to have with his challenger.