Presidential contender Hillary Clinton's proposed plan to overhaul capital gains taxes aims to foster long-term growth by taxing some short-term investments at higher rates, an aide for her campaign said on Monday.

Although details of the plan have yet to be finalized, it would create a sliding rate scale based on the length of an investment, rather than treating all assets held over a year as "long term," an aide with the Democratic candidate's campaign said.

Under her proposal, which was first reported by the Wall Street Journal, profits made by an individual selling an investment held for less than a year would continue to be taxed at regular income rates, which can rise to 39.6 percent for top earners, the Journal reported.

She would increase the maximum tax rate on capital gains made on assets held at least a year but no more than perhaps two or three years, currently 23.8 percent, to at least the 28 percent proposed by President Barack Obama, according to the Journal and a brief statement from a Clinton aide.

Clinton, the favorite to win the Democratic Party's nomination for the 2016 presidential election, has not ruled out raising it as high as the regular income tax rate, the Journal said.

She also would add additional time thresholds after which the tax rate would drop, rewarding individuals who hold assets longer. Currently, any asset held at least a year is deemed a "long-term" investment and receives the more favorable tax rate on capital gains when sold.

Clinton will give more details about the plan in a speech later this week, the Journal said.

Supporters of such a proposal say it would discourage activist investors from focusing on pushing for quick changes in a company to boost stock prices at the expense of investments, including long-term research, that take longer to bear fruit.

But some tax economists who spoke to Reuters on Monday said the proposed reform may not have much social benefit.

"My general impression is deep skepticism," Leonard Burman, director of the non-partisan think tank the Tax Policy Center and a former senior tax economist in President Bill Clinton's Treasury Department, said in a telephone interview.

"Frankly, I don't see the logic in trying to encourage people to hold assets for longer than they want to," he said, "and I don't think it will have the effects she thinks it will."

He said there were already strong incentives for individuals to hold onto assets for a long time, not least that an individual can enjoy an asset's dividends without paying capital gains tax if they hang onto the asset until their death.

Also, vast amounts of assets are held by entities, including non-profits, foreigners and retirement funds, not subject to the individual capital gains tax, Burman said.

Clinton's proposal comes as part of her plan to fight what she sees as an excessive focus on quick profits in capital markets.

In a speech last week in New York, Clinton sharply criticized risky activities on Wall Street and vowed tougher oversight in her first major economic speech of the 2016 election campaign.

Clinton's plan to revamp capital gains tax rates appears to be a shift from her position in 2008, when she last sought the party's nomination and vowed not to raise long-term capital gains tax rates above 20 percent, if at all.