Washington ? Setting the stage for a politically charged clash with Congress, President Bush on Wednesday vetoed a popular bill to expand federally funded children’s health insurance – and this time, he faces significant resistance from within his own party as well.

As with his rejection of timelines for troop withdrawals from Iraq, the president used his veto to take a politically unpopular stance. Voters’ support for billions of dollars in additional spending on children’s health insurance runs even stronger, polls suggest, than public support for ending the war in Iraq.

Yet unlike his veto of troop withdrawals, Bush may not be able to sustain his rejection of the health care measure, with Democrats casting the battle as a conflict between the president and children.

“The lines are seldom so clear on an issue,” said Sen. Max Baucus, D-Mont. “The president is wrong. The Congress is right. … The Congress is right to fight like hell to override this veto. That’s just what we will do.”

And Democrats aren’t the only ones rallying to override the veto. Many Republicans are worried that Bush’s defiant stand could imperil them in the 2008 election if the Democrats try to portray the GOP as callously indifferent to kids’ health, a message they have already begun disseminating.

The Senate passed the expansion in the State Children’s Health Insurance Program, or SCHIP, by a veto-proof margin, while the House, with a 265-159 vote, fell a little short of the necessary two-thirds majority. Now some GOP members are trying to change their fellow Republicans’ minds.

“I’m going to be making some of those phone calls to the House of Representatives to find the votes to override,” said Sen. Charles Grassley, R-Iowa, one of 18 Senate Republicans supporting the bill. “It’s disappointing to me that the president vetoed this bipartisan bill.”

A veto is the only appropriate response, the president and his allies insist, to a measure that seeks an additional $35 billion for government-financed health insurance that could be extended to many middle-class children. Bush said the bill “would move health care in this country in the wrong direction.”

“The policies of the government ought to be, help poor children and to focus on poor children,” Bush said Wednesday in Lancaster, Pa. “The policies of the government ought to be, help people find private insurance, not federal coverage. And that’s where the philosophical divide comes in.”

This is only the fourth veto for Bush during two terms as president. He twice vetoed federal spending for new lines of embryonic stem-cell research. Earlier this year, he vetoed a $120 billion war bill that included timelines for troop withdrawals, forcing Congress to give him a funding bill without restrictions.