Hillary Clinton is promising she will not raise taxes as president.

“I’m the only Democratic candidate in this race who will pledge to raise your incomes, not your taxes,” the former secretary of state and United States senator from New York said Sunday at an event in Boston.

Mrs. Clinton has already been calling, on the campaign trail, for tax simplification and less regulation of business formation and licensing. The addition of what she is now describing, formally, as a “pledge” against tax increases signals that, even before a single vote has been cast in an Iowa caucus or New Hampshire primary, the former first lady is thinking ahead to the general election and the need to win over moderate and centrist voters.

For Mrs. Clinton, such an effort will have its challenges.

The Republican candidates are almost all promising broad rate cuts from the existing 39.6% top individual income tax rate. Jeb Bush’s top proposed individual income rate is 28%, Donald Trump’s is 25%, and Ted Cruz’s is 10%.

Mrs. Clinton has yet to unveil details of her income tax plan, though she has called for a new tax credit of up to $6,000 a family to help people with the cost of caring for elderly parents. Her campaign web site says “she’ll provide tax relief to working families and small businesses.”

Mrs. Clinton has proposed to increase the capital gains tax rate on assets held for periods of between one and six years, though it is possible to argue that rate increase would amount to a tax cut because the government would collect less revenue under the higher rates. As a senator, Mrs. Clinton voted against President George W. Bush’s tax cuts in 2001 and in 2003. She has also promised to raise taxes on “carried interest” earned by managers of investment partnerships, changing the law to treat it as ordinary income rather than capital gains. That is a change on which she’s, unfortunately, been given political cover by both Jeb Bush and Donald Trump, who have taken the same position.

If experience is any guide, campaign tax promises from politicians of both parties deserve to be taken cautiously by voters. George H.W. Bush made a “read my lips” vow against tax increases then broke it. Bill Clinton ran against Mr. Bush in 1992 promising a middle-class tax cut, a promise he abandoned after the election in favor of a tax increase that hit married joint filers earning more than $140,000 a year. Mr. Clinton did later join with a Republican Congress to cut capital gainst tax rates.

Barack Obama ran in 2008 promising that “no family making less than $250,000 a year will see any form of tax increase. Not your income tax, not your payroll tax, not your capital gains taxes, not any of your taxes.” Politifact rates that a “promise broken,” given that Mr. Obama raised taxes on cigarettes and indoor tanning services and imposed a tax on people who do not purchase health insurance.

Hillary Clinton as a tax-cutting presidential candidate? If it sounds far-fetched, remember, the Clintons are nothing if not malleable. Mrs. Clinton made her no-tax-increase pledge at a “Hardhats for Hillary” event designed to highlight labor union support for Mrs. Clinton’s campaign.

“She’s dedicated her entire life to helping others,” the mayor of Boston, Martin Walsh, said in introducing Mrs. Clinton.

“I’m not going to let anybody undermine collective bargaining rights or prevailing wage standards or project labor agreements,” Mrs. Clinton said at the Boston event.

Her six years on the board of directors of Walmart, a company that is not exactly a favorite of the organized labor movement, went unmentioned.

Mrs. Clinton often talks about her granddaughter on the campaign trail. That grandchild’s other grandma — Chelsea Clinton’s mother-in-law — is Marjorie Margolies, a Democratic congresswoman from Pennsylvania who lost her seat in the 1994 election after she cast a deciding vote for Bill Clinton’s tax increase.

Republicans may want to get the “pledge to raise your incomes, not your taxes,” clip ready now for the 2018 congressional races. Or maybe a Republican Congress in 2016 would hold President Hillary Clinton to it.

Mr. Stoll is editor of FutureOfCapitalism.com.