WASHINGTON — President Obama sent Congress a $3.9 trillion budget request on Tuesday that stands as a platform for Democrats to run on in this election year, full of policies intended to invite contrasts with Republicans rather than offer compromises as he did last year, without success.

Mr. Obama’s budget for the 2015 fiscal year, which begins Oct. 1, reflects his aspirations in his waning presidency, regardless of the political reality he confronts. The document, his sixth budget, seeks to energize Democratic voters with populist proposals like a more generous tax credit for the working poor, paid for with higher taxes on the rich.

The president, as before, seeks to balance calls for spending and tax-cut policies to help the economy against measures to reduce already declining deficits. But this year his emphasis is on the investment side to address the rise in economic inequality, broaden opportunities for upward mobility and spur technological innovation.

Mr. Obama has reprised many spending and tax-cut initiatives for education from preschool through college, for roads and other public works, and for research and manufacturing centers, with many of the ideas popular among Republicans. But he has proposed to offset the initiatives’ cost by ending tax breaks for the wealthy and some corporations, which sets up a trade-off intended to put antitax Republicans in a political vise. He would bring total new revenues to more than $1 trillion over 10 years, much of it for deficit reduction.