Republicans are threatening to block the annual extension of the ban on offshore oil drilling, claiming to be building support for an effort that Democrats say smacks of a threat to shut down the government.Sen. Jim DeMint, R-S.C., has announced that 36 of the 49 GOP senators had signed a letter to Senate leaders supporting the designation of Oct. 1 - the deadline for renewing the ban for the 28th straight year - as "American Freedom Energy Day" and opposing the extension of the prohibition.The letter, addressed to both Senate Majority Leader Harry Reid and Senate Minority Leader Mitch McConnell,...