WASHINGTON — President Obama is calling on Congress to increase taxes on millionaires, reviving a proposal he first pitched last September that aims to draw sharp election-year lines between the president and the Republican opposition.

The plan, scheduled for a vote in the Democratic-controlled Senate on April 16, stands little chance of passing in Congress. But it is a prominent symbol of the efforts the president and congressional Democrats are making to portray themselves as champions of economic fairness. Republicans dismiss the idea as a political stunt with little real effect on the budget.

“We don’t envy success in this country. We aspire to it,” Obama said in his Saturday radio and Internet address. “But we also believe that anyone who does well for themselves should do their fair share in return, so that more people have the opportunity to get ahead — not just a few.”

Obama calls the plan the “Buffett Rule” for Warren Buffett, the billionaire investor who has complained that rich people like him pay a smaller share of their income in federal taxes than middle-class taxpayers.

Many wealthy taxpayers earn investment income, which is taxed at 15 percent. Obama has proposed that people earning at least $1 million annually — whether in salary or investments — should pay at least 30 percent of their income in taxes.

The push for the Buffett Rule comes as millions of Americans focus on their taxes with the approach of this year’s April filing deadline.

It also draws renewed attention to the effective tax rate of Republican candidate Mitt Romney, a millionaire who is paying 15.4 percent in federal taxes for 2011 on income mostly derived from investments.

By contrast, the top nominal rate for taxpayers with high incomes derived from wages, not investments, is 35 percent.