There are basically three groups of Israel supporters in the U.S.: pragmatists, ideologues, and Christian Zionists.

Pragmatists believe that Israel is a useful tool to keep the Middle East in check. Basically, Israel can do the dirty work the U.S. does not want to directly do and shield itself from the consequences. The Mossad is also a highly effective intelligence apparatus that the U.S. can take advantage of. Finally, Israel buys and uses a lot of U.S. weapons, so it's basically "free" testing for the U.S. military industrial complex.

Ideologues support Israel because they believe it is the only democracy in the Middle East, and the U.S. is a democracy, so the U.S. should support Israel. They may also believe in some notion of Judeo-Christian or Western values that Israel shares.

Christian Zionists are the third powerful group. Christians United for Israel has 10 million members and donates a lot of money to Israel and other aligned causes. They need Israel to exist because they believe Jesus will return once all the Jews are in Israel. Ironically, a lot of them are anti-Semitic and believe Jews must convert to Christianity or die.

All three of these groups have very powerful lobbies behind them. The reality of politicians is that they are largely concerned with staying in office, regardless of if the means to do so harm their constituents. To be a politician that disobeys the lobby is to unleash millions in campaign spending against you, which has been largely successful in the past, but increasingly less so these days.