Editor's Note: (This story originally ran December 18, 2020. It has been updated to reflect ongoing negotiations over the stimulus deal. )
(CNN) A second round of stimulus payments that was included in a coronavirus relief package passed by Congress Monday night is now at risk if President Donald Trump doesn't sign the bill.
The deal provided for $600 checks but the President indicated on Tuesday he would like that amount increased to $2,000 per person. House Republicans, however, on Thursday blocked Democrats from increasing the amount -- leaving the entire package in jeopardy.
If a deal is reached, experts say it will take at least two weeks for the Treasury to get cash into individuals' bank accounts after any new legislation is signed.
"The timing could be more challenging this time, but the IRS could likely begin to get the money out in January," said Howard Gleckman, a senior fellow at the Urban-Brookings Tax Policy Center.
In March, Congress provided individuals with $1,200 direct payments and couples with $2,400 plus $500 per child under the $2 trillion CARES Act.
As with that first round, the $600 payments included in the current legislation would start phasing out for individuals with adjusted gross incomes of more than $75,000, and those making more than $99,000 won't receive anything. The income thresholds would be doubled for couples.
It took two weeks after the first bill was passed for the IRS to start distributing the money -- but some eligible recipients still haven't received it, months later.
The payments do not go all out at once. Those whose bank information is on file with the IRS will likely get the money first because it will be directly deposited into their account. Others will receive paper checks or prepaid debit cards in the mail.
About 90 million people -- more than half of those eligible -- received their payments within the first three weeks of April after the March deal was signed. Most people had their money within two months.
Still, about 12 million eligible Americans were at risk of not getting the money at all because the IRS had no way to reach them. While most people received the money automatically, very low-income people who don't normally file tax returns had to register online before November 21 to provide their address or bank account number.
If Congress keeps the eligibility requirements the same as they were for the first round of checks, the process may be nearly as easy as hitting a button. But it could complicate things if the parameters are changed -- especially if Congress adds restrictions aside from income.
Additional checks may delay the start of the 2020 tax filing season. A second stimulus check means the agency will have to make changes to the tax return forms, some of which have already been sent to the printers.
"I believe the IRS will deliver the stimulus checks in a timely manner. It just might be at the expense of the filing season start date," said Chad Hooper, the executive director of the Professional Managers Association, which advocates for more than 30,000 non-union IRS workers.