No nation has had the privilege that we have or the high standard of living for so many as America. We are indeed responsible to God for the way we vote and participate and pray for our nation as it is "under God." Israel was specifically known by God above all nations and therefore God chose to punish them (cf. Amos 3) and not let them get away with idolatry.
The captivity in Babylon cured them of that but they had the sign of hypocrisy and legalism to boot notwithstanding. The promises in Deuteronomy 28 refer to God's covenant with Israel and not the church. The Abrahamic covenant of blessing those who bless us is still in effect.
We are admonished to not give up on our leaders but to intercede for them. There's nothing endemically wrong with infidel leaders who are moral and competent. The promise in 2 Chron. 7:14 is valid and pertinent to the Gentile nations: "If My people, who are called by My name [Christians] shall humble themselves [and seek His face, etc.]" God shall "heal the land" That's a promise you can take to the bank! America will only be punished if His people fail our country en masse. Soli Deo Gloria!