WASHINGTON — All three members of Montana’s congressional delegation criticized President Bush’s veto of legislation that would expand children’s health insurance.

Democratic Sens. Max Baucus and Jon Tester and Republican Rep. Denny Rehberg said Bush was wrong to veto the bipartisan bill, which would add $35 billion over five years to allow an additional 4 million children into the State Children’s Health Insurance Program.

The program subsidizes health coverage for 6.6 million people, mostly children, from families that earn too much to qualify for Medicaid but not enough to afford their own private coverage.

Baucus, chairman of the Senate Finance Committee, was the lead Senate sponsor of the legislation. He said he would work to find the votes to override the veto.

‘‘Whatever the reason for this inexplicable veto is the wrong choice for America’s children,’’ Baucus said Wednesday.

Bush, who quietly vetoed the bill Wednesday morning, has said the legislation is too costly and would entice people now covered in the private sector to switch to government coverage. He has proposed expanding the program by $5 billion.

Tester and Rehberg also encouraged their colleagues to override the veto.

‘‘This administration is out of touch with a lot of things, but vetoing a smart, bipartisan measure that provides health insurance to millions of children takes the cake,’’ Tester said.

Rehberg did not initially support the bill when it passed the House this summer. But he voted for compromise legislation in September after a few changes were made, including a decision to maintain eligibility for the program at 18 years old, instead of 21 years old.

‘‘The House and Senate worked out a sensible, compromise bill and the president was wrong to veto it,’’ Rehberg said.

Eighteen Republicans joined Democrats in the Senate, enough to override the veto. But in the House, supporters of the bill are about two dozen votes short of a successful override, despite sizable Republican support. A two-thirds majority in both chambers is needed.

Congress would pay for the expansion by raising the federal cigarette tax by 61 cents to $1 per pack.