By Rebecca Berg and Elizabeth Landers, CNN

(CNN) -- Fresh off a successful push on tax reform, Republicans and the White House are beginning to brace for a challenging midterm election cycle next year — with some White House allies concerned the President and the party aren't ready for the political trials ahead.

The internal tensions boiled over during a December 20 meeting in the Oval Office, where President Donald Trump and senior aides discussed political strategy for the coming year. Earlier in the day, Republican lawmakers had swarmed the South Lawn of the White House for an event celebrating their tax reform coup. But the President's team was not basking in that afterglow.

Corey Lewandowski, a former campaign manager for Trump, vented broadly about how the administration is operating — but focused his frustrations on Bill Stepien, the White House political director.

Lewandowski "wanted to stress upon the President that there's real concern that the White House and the RNC are not doing enough to prepare for the midterm election," said one former Trump campaign operative who was later informed of the meeting by a participant. "And if they're not preparing for midterm election, they're not preparing for the re-election."

The President listened and absorbed, letting the two men battle it out, according to the campaign operative. The loud disagreement spilled out after the meeting into the outer Oval, before Stepien and Lewandowski eventually stepped outside.

Stepien declined to comment and Lewandowski didn't return a call seeking comment. The New York Times first reported the disagreement following the December 20 meeting.

Growing stress

The episode reflects growing stress at the White House and among Republicans regarding the party's standing ahead of the midterms and its preparations for the likely punishing election cycle. It also raises questions over the status of the relationship between the White House and its allies at the Republican National Committee and on Capitol Hill.

When RNC officials met during the first week of November to plan for the midterm elections, "One of the biggest things that came out of that meeting was, the role of the RNC is to support the president," said one person familiar with the meeting. "That's numero uno."

If committee chairwoman Ronna McDaniel has remained committed to the President, however, party insiders believe the RNC's strategy has not always aligned with that of the White House. "Are they all on the same page? I think from the beginning, it could have been much better," said a former committee official.

A current RNC official said that Lewandowski, in his remarks to Trump, was "looking for a wedge to drive between the White House and the RNC, but there isn't one. And the political operation and fund-raising from Chairwoman McDaniel should tell the President how successful we've been over the past year."

The RNC currently enjoys a sizable cash advantage to its Democratic counterpart, with nearly $40 million on hand.

When Trump met with McDaniel earlier this month to discuss the outcome of the Alabama US Senate race and the push for tax cuts on Capitol Hill, he did not express misgivings about the RNC's strategy. Instead, according to the RNC official, "it was Steve Bannon who was feeling the pressure," with the President explicitly criticizing his former chief strategist for his role in the Alabama special election, which Republican candidate Roy Moore lost to Democrat Doug Jones.

Bannon and the President speak semiregularly.

McDaniel has delivered a memo to White House Chief of Staff John Kelly outlining the party's declining popularity with female voters, according to a source familiar with the memo. The document, which was delivered a few weeks before this month's Alabama election, was meant to be a warning that if Trump fully endorsed Moore in the wake of sexual allegations against the candidate, the President would add to the party's mounting challenges with female voters.

Ultimately, Trump ignored the warning and embraced Moore anyway.

Politico first reported the existence of the memo.

White House, GOP disconnects

The Alabama contest exposed important disconnects between the White House and party committees. Although the RNC ultimately followed the President's lead and backed the embattled Moore, the committee had initially cut ties with the candidate when Trump had not weighed in. Meanwhile, the National Republican Senatorial Committee refused to support Moore, a position at odds with the President.

The NRSC and the White House "have significantly improved that relationship in terms of how they see the political landscape" since the beginning of Trump's term, said Josh Holmes, a former chief of staff and campaign manager to Senate Majority Leader Mitch McConnell.

But there remain key areas of disagreement between the White House and congressional campaign strategists, and within the White House itself. A senior adviser to the White House said there is an urgency in the administration and the Republican Party to get a strong message out ahead of the midterms but that there is no clear organizational or implementation process in place.

"Many of us see the need to work closely together to keep the majority," the adviser said, "but there are so many factions inside and outside the White House and Congress that are out for purely their self-interest that I think there is a growing concern whether or not we can collectively get our act together."

Some Trump allies and other Republican strategists have pointed to the White House political shop as a weak link, including Stepien.

"Usually you have a political operative who has the gravitas to have interactions with the President. That doesn't exist in the administration right now," said the former Trump campaign operative. "We need to elevate a political person that the President trusts and respects."

Matt Schlapp, who served as political director in the George W. Bush White House, recalled the position as one where "you get too much credit and too much blame." Of Stepien's performance, Schlapp said he has "great comfort that he's there doing that job."

"One piece of encouragement I'd give them: Encourage the President to have a much more public role in the support of our candidates," said Schlapp. "Step up fund-raising; get the President out there."

2018 primaries

But the senior adviser to the White House said Trump or Vice President Mike Pence, the GOP's two top surrogates, likely will not dive into the primary season with in-person events — mostly sending financial resources. Once those contests have been settled, however, the White House does expect to throw its hat in the ring for certain candidates. Already, the President has privately offered his support for a few incumbents, including Sens. Roger Wicker of Mississippi and John Barrasso of Wyoming.

The President is aware of the stakes in 2018, according to multiple sources. The senior adviser said the president has been warned of "how challenging the midterm elections can be for the party in power," something that this person says has been a topic of conversation in the last few days at the White House.