House leaders in both parties were working furiously Tuesday to line up votes for and against a proposed $35 billion expansion of the State Children’s Health Insurance Program (SCHIP) that was headed to a floor vote late in the day.

“This is a defining vote for Republicans. You are either for or against health care directed by the Washington bureaucracy,’’ said Eric Cantor, R-Va., the chief deputy whip. He and other GOP leaders sought to rally their rank and file against the legislation (HR 976) at a morning caucus meeting.

They declined to offer a projected vote count but predicted Democrats would fall well short of the two-thirds majority needed to overcome President Bush’s threatened veto — a threat the White House reiterated Tuesday morning. “We will have enough votes to sustain a veto,” Cantor said.

Democratic leaders did not directly dispute that claim. House Speaker Nancy Pelosi, D-Calif., said the battle has just begun.

“We are hoping to galvanize the support of the American people behind this legislation,’’ she said, referring to a “drumbeat across America that extends to Republicans across America.” She said Bush is isolated on this issue. “We’ll see how long he can hold a veto-proof majority,’’ she said.

In a statement of administration policy, the White House said Bush supports SCHIP’s “original purpose of targeting health care dollars to low-income children who need them most. However, the current bill goes too far toward federalizing health care and turns a program meant to help low-income children into one that covers children in some households with incomes of up to $83,000 a year.”

The statement called on Congress to pass a simple extension of SCHIP before the program’s funding authorization expires Sept. 30.