Obamacare was supposed to be President Barack Obama’s legacy. But it’s looking like a political millstone.

The mammoth and unpopular health insurance overhaul weighed down Democrats in 2010 when Republicans helped turn seniors to their side.

And now Democrats have unexpectedly had to play defense over Obamacare’s Medicare cuts even as Mitt Romney picked Congressman Paul Ryan as a vice-presidential running mate and drew attention to unpopular Republican plans that cap future Medicare spending.

Central to the Republican attack: Obamacare cut $716 billion in anticipated Medicare spending over a decade. Republicans are driving the message home in TV ads and robocalls bashing Democrats.

“This could cost us the election,” Kelly Ward, the Democratic Congressional Campaign Committee’s political director wrote in a fundraising email last week. “We have to get the facts out to voters immediately.”

The first Democratic talking point: Ryan’s Medicare plan signed off on the very Obamacare cuts that Republicans are now bashing.

Obama’s campaign defended itself Friday in an ad that said the “the non-partisan AARP says Obamacare ‘cracks down on Medicare fraud, waste, and abuse,’ and strengthens guaranteed benefits.”

On Saturday, Ryan kept up the anti-Obamacare pressure in his first presidential-campaign trip to Florida, where he spoke at the Republican-friendly retirement community The Villages, along with his mom, a part-time Lauderdale-by-the-Sea resident.

The Obamacare cuts to Medicare haven’t yet reduced services and are structured in such a way that they’re not supposed to hit benefits, at least in the short term. Medicare spending is actually forecast to increase in 2012 at a higher annual rate than the average annual increase over the prior five years.

The cuts are targeted at insurance companies and hospitals, not beneficiaries. Whether they’ll reduce services in future years is an open question.

Romney’s ad, however, makes it sound as if the cuts directly hit current Medicare recipients.

“You paid in to Medicare for years — every paycheck. Now, when you need it, Obama has cut $716 billion dollars from Medicare,” the ad says.

But beyond the $716 billion number, most facts and figures might not matter. Republicans and Democrats say voters will make their decision on whom they trust more, not whose plan has every single detail spelled out. Neither side is willing to admit that Medicare, assuming taxes aren’t raised, will have to experience actual cuts in future benefits.

Republicans charge Democrats with “stealing” from Medicare to expand government, namely Medicaid, the expensive health insurance program for the poor that would be capped under Romney and Ryan, whose plan trims about $810 billion from the program over a decade.

Democrats charge Republicans with “shredding the social safety net” to pay for tax cuts for millionaires. Medicaid, they note, has paid for up to two-thirds of daily nursing home expenses in Florida, where 3.3 million people receive $21.4 billion of the program’s benefits.

The battle lines are thus clearly drawn over who’s plan is worse for retirees. And the winner of that argument could win the White House.

It’s an issue of great importance in Florida, a must-win state for Republicans where nearly 3.4 million annually receive $25.2 billion in Medicare services. More than half the electorate in Florida is 50 or older.