CONCORD, NH: Mounting attacks on Steve Forbes' flat tax proposal indicate the GOP presidential field feels threatened and that "his political life (is) on the line," as Forbes himself told New Hampshire voters Thursday. His bounteous bank account and purist approach to the flat tax (17 percent with no deductions) has thrust him into the national spotlight and put Forbes within striking distance of front-runner Bob Dole. Enter GOP rivals, stage left. It's a "nutty idea," lagging candidate Lamar Alexander is now saying. It "will cost us the election," Pat Buchanan told voters in Louisiana, echoing Bob Dole's position. Phil Gramm weighed in, "There's no way we can defend [his] system." A Dole-sponsored tax commission released its report Wednesday endorsing the flat tax, but left out key specifics such as what percentage of income should be taxed and what deductions should be allowed. That allowed Dole wiggle room while still showing him favoring a simpler, flatter tax code. Forbes has held his ground, saying the lost deductions would be compensated for by higher savings rates and lower interest rates. "Please go beyond what they say you will lose," he asked voters. But he too is maneuvering. While Forbes has downplayed the importance of balancing the budget, he announced Thursday that if elected he'd propose a constitutional amendment to ensure it.