WASHINGTON — Here is what Democrats should learn from their party’s loss in a special House election in Florida last week: Wishy-washy won’t work.

Republicans are obviously going to make opposition to the Affordable Care Act the main theme of their campaigns this fall. Democrats will be better off if they push back hard — really hard — rather than seek some nonexistent middle ground.

The contest between Democrat Alex Sink and Republican David Jolly in Florida’s 13th Congressional District was almost like a laboratory experiment. The House seat was held for decades by the late C.W. “Bill” Young, a Republican, but voters are evenly balanced between the two parties. Sink was better known, having narrowly lost a race for governor in 2010; Jolly had deeper roots in the community. Neither displayed an overabundance of charisma.

Jolly’s narrow victory — he won by about 3,500 votes out of about 184,000 — is not a death knell for the Democratic Party’s prospects come autumn. But it does suggest how Democrats should not run in close races. Jolly has to run again in November, and if Sink gets another shot at him, I’d suggest she do things a bit differently.

Jolly ran what amounted to a one-note campaign: Repeal Obamacare. Given the large number of retirees in the district, he hammered away with the untrue GOP claim that the health care law somehow weakens Medicare. Jolly followed his party’s leadership in declining to propose a coherent set of different health insurance reforms to replace the ACA.

Sink defended President Obama’s signature achievement but added the caveat that she wants to “keep what’s right and fix what’s wrong.” This is the course of action most voters would prefer, according to opinion polls. But as a campaign position, “fix what’s wrong” sounds weak and equivocal — especially when contrasted with the bold GOP promise of repeal.

That promise, of course, is dishonest. House Republicans have already staged 50 meaningless votes to repeal all or part of the Affordable Care Act, knowing that none of the measures had a prayer of making it through the Senate. Even if the GOP were to control both chambers, Obama would veto any repeal bill that reached his desk. Democrats facing close contests this fall should play offense on Obamacare, not defense. They should tell voters that the ACA is a landmark achievement — the biggest expansion of access to health care in decades, fulfilling a long-held progressive dream. They should accuse their GOP opponents of playing voters for fools by cynically pretending that repeal is around the corner. Democrats should talk about what’s right with the ACA. They should talk about the millions of formerly uninsured Americans who now have coverage. They should talk about the millions of others who are covered for the first time under Medicaid. They should talk about the young people who are able to be covered under their parents’ policies. They should talk about the diabetics and cancer survivors who now cannot be denied coverage because of their conditions.

Democrats have long taken the position that no one should have to declare bankruptcy because of illness, that no one should have to choose between paying for medicine or the mortgage. If Democrats can’t proclaim these beliefs with pride, why on earth are they running? Being positive rather than negative on Obamacare won’t be enough, however. Democrats also need to give their voters a compelling reason to go to the polls — and that means explaining, in the most urgent terms, just what is at stake.

Today’s Republican Party is not “conservative” by any reasonable definition of the word. It is a radical party seeking to dramatically alter the social compact by which we have lived for decades. Republicans, if they could, would slash Social Security benefits and turn Medicare into a voucher program. They’re the ones who threaten the safety net for seniors, not Democrats. Republicans refuse to invest in our decaying infrastructure. They want to do away with government regulation that has given us cleaner air, healthier food, safer workplaces. They seek ultimate control over women’s reproductive rights and have already made it oppressively difficult to terminate a pregnancy in many states. Instead of comprehensive immigration reform, they propose “self-deportation.”

The Republican agenda is dangerously wrong for America. The majority coalition that elected Obama twice isn’t accustomed to voting in midterm elections. Democrats need to explain why this year has got to be an exception.

Eugene Robinson is a Washington Post columnist. His email address is eugenerobinson@washpost.com.