The discussions are preliminary, and are likely to stretch beyond the end of October, when Boehner’s resignation from Congress takes effect. But if the talks can produce top-line numbers for domestic and defense spending, that could help the GOP-led Congress avoid future showdowns over government spending like the standoff that will loom in mid-December.

“We’d like to settle a top-line for both years so that next year, we could have a regular appropriations process,” McConnell said Tuesday. “The president and Speaker Boehner and I spoke about getting started into discussions last week, and I would expect them to start very soon.”

A Boehner aide confirmed the talks, saying the men “discussed the need to get moving on the budget process.”

A major budget deal could be elusive, however. Democrats and many Republicans are eager for a broader agreement that would lift federal spending for domestic programs — the top demand of congressional Democrats — while hawkish GOP lawmakers want more money for defense programs.

But many conservatives want to keep strict spending caps in place, praising them as a measure of fiscal discipline. In exchange for increasing some key spending levels, McConnell will insist on offsetting those costs and will take a hard line against any tax hikes.

The December spending showdown will also serve as a major test for House Majority Leader Kevin McCarthy (R-Calif.), the likely successor to Boehner who is promising to take a more aggressive approach with the Senate and the White House in his bid for speaker. Because the budget talks appear highly unlikely to conclude before Boehner leaves Congress, McCarthy would immediately have to take the Ohio Republican’s place in the high-stakes negotiations.

The initial phone conversation with Obama, Boehner and McConnell occurred Sept. 17, shortly before the president met with Senate Minority Leader Harry Reid (D-Nev.) and House Minority Leader Nancy Pelosi (D-Calif.) at the White House, according to congressional aides and a White House official.

During the call with Obama and GOP leaders, McConnell proposed that the budget negotiations be limited to just himself, the president and Boehner — a suggestion that was discussed with Reid and Pelosi during a meeting with Obama that followed the phone call, Democratic sources said.

The two Democratic lawmakers were already slated for an Oval Office meeting that day; they had agreed on a strategy to pursue a stopgap bill and negotiate a spending bill that reverses sequestration after the immediate threat of a shutdown has passed.

Boehner and Obama pushed back on McConnell’s suggestion, the sources said, insisting that congressional Democrats be a part of the negotiations. In the days after that conversation, Obama again spoke to McConnell and Boehner to tell them that the Democrats — including the administration and congressional Democratic leaders — were ready to begin budget talks, according to the White House official.

“Sen. Reid will welcome Sen. McConnell to the negotiating table whenever Sen. McConnell is ready,” Reid spokeswoman Kristen Orthman said. “Sen. Reid has been urging Republicans to begin this process for months instead of waiting for the last minute. But better late than never.”

A spokesman for McConnell declined to comment on the private phone conversations with the majority leader, Boehner and Obama.

“There’s stuff going on as we speak, on the staff level,” Reid said Tuesday of budget negotiations. “If there are any talks, I’ll be invited.”

Congress is poised to avert the latest threat of a government shutdown, with the Senate set to pass a short-term spending measure Wednesday morning that funds the government through Dec. 11. Then, the House is expected to pass the spending bill and send it to Obama for his signature just in time to avoid a funding lapse for federal agencies.

Senate Democrats have blocked all spending bills from coming to the Senate floor this year, due to the dispute over the current funding levels. But rather than top-line figures, September’s fiscal showdown centered on the controversy surrounding Planned Parenthood, with conservatives pushing to defund the women’s health organization after allegations surfaced that Planned Parenthood profited from sales of fetal tissue.

Though the current short-term spending bill doesn’t defund the group, Republican leaders warned they weren’t backing off their pressure on Planned Parenthood. They pointed to congressional investigations underway and indicated that they were preparing to use a powerful procedural weapon known as reconciliation to get a measure defunding the group — as well as unraveling key aspects of the president’s health care law — to Obama’s desk.

“This bill hardly represents my preferred method for funding the government,” McConnell said Tuesday morning. “But it’s now the most viable way forward after Democrats’ extreme actions forced our country into this situation.”

Sarah Wheaton contributed to this report.