The House has passed a bill that would gradually increase the child tax credit and make it available to more families with higher incomes.
But millions of low-income families would lose the $1,000-a-child credit in 2018, when enhancements championed by President Barack Obama are set to expire.
The bill also aims to make a dent in illegal immigration by prohibiting people without Social Security numbers from claiming a portion of the credit reserved for low-income families.
The White House has threatened to veto the bill, saying it favors high-income taxpayers over the poor, while adding $90 billion to the budget deficit over the next decade.
House Republicans say the bill strengthens the tax credit by increasing it as inflation rises, and by making it available to more middle-income families.