WASHINGTON — Hours before the Monday deadline for Americans to sign up for ObamaCare in order to have coverage by Jan. 1, a Senate Democrat warned the plan could soon collapse of its own weight.

“If it’s so much more expensive than what we anticipated and if the coverage is not as good as what we had, you’ve got a complete meltdown at that time,” Sen. Joe Manchin (D-W.Va.) told CNN.

After a series of delays and stopgap fixes by the administration, Manchin is pushing to delay the ObamaCare individual mandate — which the administration has refused to do across the board, though it is deferring the mandate for people who lost their individual health plans.

“This transitional year gives you a chance to adjust the product for the market,” Manchin said.

“It falls under its own weight, if the cost becomes more than we can absorb,” he added.

“Don’t miss the deadline: Sign up for health insurance ASAP,” the White House tweeted yesterday from President Obama’s official account. In an e-mail to supporters, Obama, who is on vacation in Hawaii, noted that 500,000 people have signed up on the trouble-plagued HealthCare.gov Web site in the first three weeks of December.

But even Obama didn’t seem quite sure about the deadline. Health and Human Services officials put the deadline to get covered at 11:59 p.m. on Monday.

In a letter released by the White House, the president told Americans if they sign up “before December 23rd, you can be covered on the first day of the New Year.”

That phrasing would appear to exclude Monday — even though the administration has been banking on last-minute enrollments.

Even as Obama issued a final plea for people to sign up, the latest delay — letting people avoid a penalty if they’ve lost their low-end individual plan — creates an incentive to hold off.

“Everybody who signs up and had a high deductible policy should go and cancel today,” said Sen. Tom Coburn (R-Okla.). “You can’t fix this mess.”

People who miss the deadline can still sign up for coverage, but it won’t start until February.