Is President Obama finally going to bring some fire to the midterm elections? White House political advisers are considering running national ads that would tie Republicans to the Tea Party movement, warning unenthusiastic Democratic voters that putting the GOP back in power this fall would be dangerous. But the Democratic campaign committees in the House and Senate are opposed to the move because it would further nationalize the election, something they want to avoid in a time of high unemployment and low presidential popularity. Supporters of the ad buy, which would likely air on cable TV, say it would redefine the debate, showing how high the stakes are with the nomination of very conservative Tea Party candidates. The White House is denying the Times story; but the Times is standing by it.