WASHINGTON/NEW YORK, Dec 9 (Reuters) - For the Republican Party, a tough job just got a little tougher thanks to Donald Trump. For years, Republicans, backed by some influential donors, have carefully cultivated the vote of Jewish-Americans who regard the security of Israel their top concern. Less than a week ago, a bevy of presidential candidates stood before a group of Jewish conservatives in Washington, asking for their support and declaring their solidarity with the Jewish state. But Trump's proposal to bar the entry of all foreign Muslims into the United States has upended some of those efforts, prominent Republicans...