WASHINGTON — President Barack Obama said Wednesday he has seen no evidence that national security was threatened by the widening sex scandal that ensnared his former CIA director and top military commander in Afghanistan.

In his first postelection news conference, Obama also reaffirmed his belief that the U.S. can't afford to continue tax cuts for the wealthiest Americans, a key sticking point in negotiations with Republicans over the impending "fiscal cliff." He said, "The American people understood what they were getting" when they voted for him after a campaign that focused heavily on taxes.

And he defiantly told critics of U.N. Ambassador Susan Rice, a potential candidate to lead the State Department, that they should "go after me" — not her — if they have issues with the administration's handling of the deadly attacks on Americans in Benghazi, Libya. His words were aimed at Republican Sens. John McCain and Lindsey Graham, who have vowed to block Rice's potential nomination.

The tangled email scandal that cost David Petraeus his CIA career and led to an investigation of Gen. John Allen has disrupted Obama's plans to keep a narrow focus on the economy coming out of last week's election. And it has overshadowed his efforts to build support behind his re-election pledge to make the wealthy pay more in taxes in order to reduce the federal deficit.

Obama said he hoped the scandal would be a "single side note" in Petraeus' otherwise extraordinary career.

Petraeus resigned as head of the CIA last Friday because of an extramarital affair with his biographer, Paula Broadwell, who U.S. officials say sent harassing emails to a woman she viewed as a rival for the former general's affection. The investigation revealed that that woman, Jill Kelley, also exchanged sometimes-flirtatious messages with Allen.

Obama brushed aside questions about whether he was informed about the FBI investigations that led to the disclosures quickly enough. White House officials first learned about the investigations last Wednesday, the day after the election, and Obama was alerted the following day.

"My expectation is that they follow the protocols that they've already established," Obama said. "One of the challenges here is that we're not supposed to meddle in criminal investigations and that's been our practice."

Turning back to the economy, the president vowed not to cave to Republicans who have pressed for tax cuts first passed by George W. Bush to be extended for all income earners. Obama has long opposed extending the cuts for families making more than $250,000 a year, but he gave into GOP demands in 2010 when the cuts were up for renewal.

That won't happen this time around, he said Wednesday.

"Two years ago the economy was in a different situation," Obama said. "But what I said at the time was what I meant. Which was this is a one-time proposition."

The president and Congress are also seeking to avoid across-the-board spending cuts scheduled to take effect because lawmakers failed to reach a deal to reduce the federal deficit. Failure to act would lead to spending cuts and higher taxes on all Americans, with middle-income families paying an average of about $2,000 more next year, according to the nonpartisan Tax Policy Center.

Obama said he was "open to new ideas" but would not allow current tax rates to continue for the top 2 percent of wage earners, drawing a line for Republicans who say they will not tolerate any tax rate increases. Asked if the tax rates for the rich had to return to Clinton-era levels, Obama indicated he was open to negotiations.

On another topic, Obama said it was his "expectation" that a comprehensive immigration reform bill would be introduced "very soon after my inauguration."

His support among Hispanics was one of his keys to victory over Republican rival Mitt Romney, who staked out conservative positions on immigration changes during the GOP primaries. Obama failed to make progress on comprehensive immigration changes during his first term but said he planned to "seize the moment" after his inauguration.

The White House is already engaged in conversations with Capitol Hill.

Obama said the legislation should make permanent the administrative changes he made earlier this year that allow some young illegal immigrants to remain in the country legally. He said that the overall bill should include a "pathway to legal status" for the millions of immigrants who are in the U.S. illegally but and haven't committed crimes unrelated to immigration.

Before tackling immigration, Obama will have to face the departure of several key Cabinet secretaries and White House staffers. Among those expected to leave are Treasury Secretary Timothy Geithner and Secretary of State Hillary Rodham Clinton.

Rice, the U.S. ambassador to the United Nations, and Massachusetts Sen. John Kerry are the leading candidates to replace Clinton. Rice is a favorite of the president, but she has faced intense criticism for her role in the initial administration response to the deaths of four Americans, including the U.S. ambassador to Libya, during an attack

"When they go after the U.N. ambassador, apparently because they think she's an easy target, then they've got a problem with me," Obama said. "And should I choose, if I think that she would be the best person to serve America, in the capacity of the State Department, then I will nominate her. That's not a determination that I've made yet."