Mr. Obama will also step up his efforts to draw contrasts between the parties, in particular by pounding away on his call for extending the expiring Bush-era tax cuts, except for “millionaires and billionaires.” Republicans want the tax cuts extended for people of all income levels, not just incomes below $200,000 for individuals and $250,000 for families, as the president has proposed.

Republican strategists remain confident of the party’s prospects for big gains in November, even as they acknowledge that they are unlikely to win the Senate race in Delaware after the victory in the Republican primary there of Christine O’Donnell, a Tea Party-backed candidate with a long record of controversial statements, over Representative Michael N. Castle, a moderate and popular former two-term governor.

Also last week, Alaska’s Senate race was upended when Senator Lisa Murkowski, who lost the Republican nomination to a Tea Party adherent, Joe Miller, mounted a write-in candidacy against him, saying, “Alaska is not fair game for outside extremists.”

“While we may have a handful of nominees out of the mainstream, the American people have come to the conclusion this administration and this Congress are out of the mainstream,” said John Weaver, a Republican consultant.

In 1994, Democrats were in power and similarly took hope when Republican primaries yielded candidates deemed too far right for the general election. Yet the wave against Democrats that year was strong enough to carry those newcomers into office and put Republicans in control of Congress for the first time in 40 years.

Except for Ms. O’Donnell in Delaware, Republican nominees that Democrats like to showcase as extremists — including in Senate races in Nevada, Colorado, Kentucky and even blue-state Connecticut — are even with their Democratic rivals in polls or ahead.

And even as the White House maps the final campaign push, advisers are distracted by the expected exit of the chief of staff, Rahm Emanuel, to run for mayor of Chicago. Mr. Emanuel, who as a member of Congress helped engineer the Democratic takeover of the House in 2006, is among his party’s foremost strategists when it comes to Congressional elections.