Talks to avert the year-end “fiscal cliff” appeared on the verge of an impasse Wednesday morning after late-night attempts to prevent negotiations from breaking down yielded no significant progress.

President Obama’s chief negotiator, Rob Nabors, rushed to the Capitol late Tuesday to meet with top aides to House Speaker John A. Boehner (R-Ohio) hours after the two leaders had a phone call described by Boehner in tepid terms.

“The president and I had a deliberate call yesterday, and we spoke honestly and openly about the differences that we face,” Boehner told reporters Wednesday morning. “But the president’s called for $1.4 trillion worth of revenue. That cannot pass the House or the Senate.”

Boehner’s characterization of the call was less positive than how he had described his face-to-face meeting with Obama on Sunday at the White House. On Tuesday, Boehner described that meeting as “cordial.”

Despite the differences, Boehner said he remained hopeful on Wednesday: “I was born with a glass half full. I remain the most optimistic person in this town, but we've got some serious differences.”

The speaker’s public comments followed his weekly meeting with fellow House Republicans, during which he advised colleagues not to make major holiday plans, according to Rep. Peter King (R-N.Y.), who attended the meeting.

Democrats also sounded less hopeful Wednesday of striking a deal by Christmas. Rep. Chris Van Hollen (D-Md.) said that both sides need to craft the frameworks of a deal by the end of this week in order to ensure its passage by the holiday. But Van Hollen, the ranking Democrat on the House Budget Committee, told reporters at a Wednesday morning breakfast hosted by the Christian Science Monitor that he didn't see that timetable “as a high probability."

Aides familiar with Tuesday night’s meetings said no significant progress was made between Nabors and congressional negotiators. The meeting came after White House officials complained that they were still waiting for Republicans to produce an offer that includes higher tax rates for the wealthy, a major focus of Obama’s reelection campaign.

The sniping comes just three weeks before nearly $500 billion in automatic tax hikes and spending cuts are set to take effect, potentially triggering a new recession. Earlier in the week, negotiations appeared to be intensifying after Boehner’s visit to the White House.

Neither the White House nor Republican leadership aides would provide details of the latest GOP plan, but Republican sources close to the talks said the offer made no concessions on the central issue of higher tax rates for the wealthy.

More on the "fiscal cliff" impasse here.

Rosalind S. Helderman contributed to this report.