BOCA RATON, Fla. — All but certain now that his Republican opponent will be Mitt Romney, President Obama has made his proposed “Buffett Rule” minimum tax for the wealthiest Americans like Mr. Romney a centerpiece of his re-election campaign, defying the political risk of being seen as a tax-and-spender by wary voters.

With a rousing speech on Tuesday to a receptive university audience of about 5,000 in this battleground state, Mr. Obama defined the coming contest as a clash of philosophies: His argument that tax fairness and the common good demand the richest Americans pay at least as much as middle-income taxpayers do, contrasted with Republicans’ opposition to any tax increases as job killers and class warfare, even at the cost of deep cuts in domestic programs.

While voters have not often rewarded candidates who advocate tax increases, Mr. Obama and his campaign advisers, in league with Democrats in Congress, express confidence that voters are on their side, with polls showing that Americans overwhelmingly agree that wealthy taxpayers should pay more and that they favor spending for programs like education, research and health care.

The Democrats’ offensive, not coincidentally, is tied to the likelihood that Mr. Romney will be the nominee. Even as Mr. Obama was speaking at Florida Atlantic University, Mr. Romney’s chief Republican rival, former Senator Rick Santorum, was announcing that he is leaving the race. In past months Mr. Romney’s Republican rivals had caricatured him as a Wall Street investor raking in millions with job-cutting corporate takeovers and slashing his taxes through loopholes and Swiss bank accounts, so he was hardly likely to get off more easily with Democrats.