The Democratic presidential candidate Martin O’Malley wants to raise capital gains taxes. His rival Bernie Sanders seeks to tax stock trades and increase personal income tax rates.

Indeed, Mr. Sanders’s plan harks back to the Eisenhower-era top rate of over 90 percent — more than twice today’s level. Asked whether that rate is obviously too high, Mr. Sanders responded simply, “No.”

Their stances partly reflect attempts to outflank Hillary Rodham Clinton in the Democratic race for the presidential nomination. Mrs. Clinton herself is considering a friendly think tank’s advice to raise taxes on inherited wealth and close loopholes to collect more from affluent Americans.

But they also reflect a broader shift in tax politics that is rippling through the Republican world, too. Pressure to raise taxes, at least on the wealthy, is rising.