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Evoking the investment in American infrastructure by Presidents Franklin D. Roosevelt and Ronald Reagan, Hillary Clinton on Monday unveiled the most sprawling — and costliest — government program of her campaign to date.

Mrs. Clinton said her five-year, $275-billion federal infrastructure program was aimed at creating middle-class jobs while investing heavily in improving the country’s highways, airports and ports. Bridging the “infrastructure gap” between the United States and developing nations like China would also eliminate red tape and fuel overall economic growth, she said.

“Workers can’t get to work, congestion keeps parents stuck in traffic, floods threaten our cities, and airports leave travelers stranded for hours or even days at a time,” the campaign wrote in a fact sheet about the proposal.

The plan is the start of a monthlong shift by the campaign to a focus on domestic proposals based on what it calls Mrs. Clinton’s “jobs agenda.” Since she declared her candidacy in April, Mrs. Clinton has proposed policies to regulate the financial industry, combat climate change, help ease the heroin epidemic and provide tax relief to families carrying for sick relatives, among other things.

The jobs portion of her policy platform, which she will present to voters on the campaign trail in Iowa, New Hampshire and elsewhere this month, amounts to the most in-depth and expensive of her domestic proposals, with the infrastructure plan being the centerpiece.

“Research has found that infrastructure investment creates overwhelmingly middle-class jobs in construction and manufacturing sectors,” said Alan B. Krueger, a professor of economics and public affairs at Princeton and a former chairman of President Obama’s Council of Economic Advisers.

The plan would increase federal funding for infrastructure by $275 billion, with $250 billion from direct public investment and $25 billion to create a national infrastructure bank. The campaign said the plan would be fully paid for by reforming the business tax code, but did not provide additional details. Mr. Krueger said parts of the plan, such as the reauthorization of the “Build America Bonds” program to drive investment in infrastructure, would be set at a rate that makes them revenue neutral with respect to tax-exempt bonds. He added that the infrastructure bank would be highly leveraged.

Republicans have criticized Mrs. Clinton for proposing costly government programs with only vague explanations of how she would pay for them. A spokesman for the Republican National Committee, Michael Short, called her latest plan the continuation of a “spending binge already at a trillion dollars and counting.”

Mrs. Clinton has repeatedly told voters that she will not raise middle-class taxes. “I’m the only Democratic candidate in this race who will pledge to raise your incomes, not your taxes,” Mrs. Clinton often says, a reference to Senator Bernie Sanders’s health care plan that she says amounts to a tax increase.

Each federal investment of $1 billion in infrastructure leads to the creation of 13,000 jobs, and each dollar invested in improving highways, railways and bridges leads to an estimated $1.60 increase in gross domestic product the following year, according to the campaign, using data from the Council of Economic Advisers.

At a “Hard Hats for Hillary” rally in Boston on Sunday, Mrs. Clinton called the plan “a down payment on our future” and said she would go further than the legislature currently being considered by Congress.

“I don’t have to tell you what a sorry state we’re in,” Mrs. Clinton told a crowd made up largely of labor union members. “Sometimes people say ‘Well, yeah, yeah, we’ll get around to it. We don’t have time.’ We’ve got to do this now.”

Damon Silvers, director of public policy at the A.F.L.-C.I.O., among the country’s most influential labor unions, praised the plan, saying “Secretary Clinton is exactly right to call her plan a ‘down payment.’”

For some activist groups, however, the plan did not go far enough.

“It’s a trillion-dollar problem,” said Deepak Bhargava, executive director of the Center for Community Change Action, a group that works to help low-income people. “We need much bolder proposals from the leading Democratic candidate.”

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