It's now or never for tax reform this year.

On Tuesday, the White House director of the National Economic Council, Gary Cohn, underscored the urgency to complete a tax overhaul so that Congress can tackle other top priorities next month.

"December is going to get very, very crowded," Cohn said speaking at The Wall Street Journal's CEO Council conference.

Earlier this year, lawmakers punted on a number of critical issues until the first and second week of December, including a possible government shutdown. Congress has until midnight on December 8 to pass a spending bill, or risk the federal government running out of money.

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"It's really important to get it done," Cohn stressed. "We've gotta get taxes done this year."

Republicans haven't been shy about hiding their motivation to get tax reform done in the weeks ahead. If they fail to do so, their donors could wind up abandoning them, imperiling their chances of keeping the majority in 2018.

Congress hasn't been able to move any significant changes to the tax code in more than 30 years.

CEOs are certainly losing confidence in the prospect, given how far apart the House and Senate proposals are.

Roughly 58% of CEOs surveyed at the conference during Cohn's panel expressed pessimism as to whether Washington can get tax reform done by the end of 2017.

But Cohn waved off doubts, and pointed to the progress that both chambers are making on their respective legislative proposals.

House lawmakers are expected to vote on their tax reform bill this week. In the Senate, Republicans have begun marking up their own tax plan in the finance committee, a process that could take several days.

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Cohn said he expects both chambers to make significant progress by the end of this week, before lawmakers head to their home states for Thanksgiving recess.

"So by the end of this week... we will have the bill through the House and we will have the Senate Finance committee done," said Cohn. "The only thing left to do is get the full Senate vote on the bill."

He did raise the possibility that both sides would have to reconcile the bill in a conference committee, which would require both chambers to vote on the bill for a second time.

Still, Cohn downplayed differences between the two plans.

"The question should be: 'Do the bills deliver middle-income tax relief?' And the answer to both is, 'Yes'. How they get there is different," said Cohn.

Later Tuesday, Senate Republicans moved to include a repeal of Obamacare's individual mandate in the revised version of the chamber's tax overhaul. That's expected to be released by the Senate finance committee later today.

Cohn suggested on Tuesday that the administration would press ahead regardless of whether enough senators vote to repeal the individual mandate.

"We are still plotting ahead on tax reform whether [that] happens or it doesn't happens," said Cohn.

--CNN's Lauren Fox contributed to this report.