WASHINGTON, Sept. 21 — George W. Bush ran for the White House as a compassionate conservative. Now he is portraying himself as a conservative of a different stripe — the fiscal kind.

Mr. Bush is headed into a spending battle with Congressional Democrats with the two branches of government as divided on the issue as they have been since the federal shutdown of 1995. The president has threatened to veto 10 of the 11 appropriations bills that have passed the House, as well as legislation expanding a popular children’s health care program. On Monday, he will step up the fight with a speech accusing Democrats of fiscal irresponsibility.

That might not sound altogether unexpected, given that Democrats, whom Mr. Bush accuses of “working to bring back the failed tax-and-spend policies of the past,” now run Congress. But the president’s new tough talk on spending has also caught some fiscal conservatives in his own party off guard. After years lambasting Mr. Bush for letting government spending run out of control, they wonder what has gotten into him.

“I don’t know what kind of epiphany he had,” said Representative Jeff Flake, Republican of Arizona, who fought on Capitol Hill to rein in the federal budget, to little avail. “I’m just glad he had it.”