By Alister Bull

WASHINGTON (Reuters) - President Barack Obama, seizing on Republican plans to slash spending that the White House sees as a potent vote winner for Democrats in this year's election, will use a speech on Tuesday to reinforce his message that his opponents favor the rich.

In a week in which Republican Mitt Romney is expected to extend his lead in the race for his party's nomination to confront Obama in the November 6 election, the president will also speak at a conference on women in the economy, White House officials said.

A new poll on Monday showed Obama opening a large lead over Romney among women in key election swing states.

White House officials were not surprised by the poll's findings after recent remarks about birth control by Republican candidates on the campaign trail, which they said seemed like an echo from the 1950s.

Romney has said he would end federal funding for the Planned Parenthood women's health organization that provides abortion services and Rick Santorum, his chief rival for the Republican nomination, has called contraception morally wrong.

Obama, speaking at a joint press conference with Canadian Prime Minister Stephen Harper and Mexican President Felipe Calderon, said he would "cut folk some slack for now" by not commentating on the Republicans until they had a nominee.

Romney hopes to all but wrap the race up this week by winning election contests in Wisconsin, Maryland and Washington D.C., pressuring Santorum to stand aside to unite the Republican party in the goal of defeating Obama in November.

But senior White House officials said the president would use a speech in Washington on Tuesday to press his priorities on taxes, the future of federally provided Medicare healthcare, and a defense of the middle class that Obama hopes will peel vital blue collar voters away from Republicans.

They told reporters during a briefing on Monday that the goal of fairness in the tax system captured by Obama's so-called Buffett Rule would be a core part of the message, ahead of a tax vote in the Democratic-controlled Senate later this month.

Named after billionaire Obama backer Warren Buffett, who complained last year that he payed a lower tax rate than his secretary, Obama says the Buffett Rule is a standard that should guide Congress as it seeks to tackle the U.S. deficit and debt.

Republicans in the U.S. House of Representatives last week passed a budget proposal to aggressively shrink the U.S. deficit that Democrats complain puts the burden unfairly on the poor and middle class, while granting tax breaks for the rich.

The Republican plan caps discretionary spending on things like education and infrastructure and cuts spending on welfare programs for the poor including for food stamps and housing.

The blue print also proposes broad tax reform, including the closure of loopholes to raise revenue, while advocating a simplification in the tax code that would include lowering the top rate of tax to 25 percent from 35 percent at present.

"Should we ask middle-class Americans to pay even more at a time when their budgets are already stretched to the breaking point? Or should we ask some of the wealthiest Americans to pay their fair share?" Obama asked in his weekly address on Friday.

(Reporting by Alister Bull; Editing by Lisa Shumaker)