Yes, the US should apply serious pressure to get Israel to comply to the UN resolutions and the 4th Geneva convention. Without US aid the illegal settlments in the occupied territories simply would not be viable as 50% of US economic aid to Israel is spent on supporting the settlements economically (this does not include the vast amount of money spent on defending the settlments).
Mooka, there is no simple answer to your question. There are just under 6 million Jews in the US (according to the Jewish virtual library), which makes it the country with the largest Jewish population in the world (even larger than Israel). In a country of about 250 million people this is not however a very large voting bloc. Some people will tell you that there is an all powerful and shady Jewish lobby (zog), who exert immense pressure on the US government, others will tell you that no such lobby exists. The truth is there is a reasonably powerful Zionist (Zionist and Jewish are not interchangable and the Zionist lobby is not exclusively Jewish) lobby (though nothing like as powerful as the oil, gun, big buisness, tobbacco, etc. lobbies), which lobbies politicians in the traditional manner and promotes their point of view in the US press.
Another factor that is nearly always overlooked by people who like to invoke the zog conspiracy are the dispensationalists. These are right-wing fundamentalist Christians (IIRC numbering about 70 million) who believe that the aliyah (Jewish return to Israel) is part of a biblical propehecy that signals the ‘end times’ (this idea was first formulated in England in the 19th century by the cult-like Plymouth Brethren). They make up a very powerful political force in the US as they will always vote along pro-Israeli lines (most Israelis themselves view these people as nutcases or even as antisemtic, though some US Jewish groups like the ADL have forged alliances with them).
There are of course other reasons for the US’s support of Israel and they were pretty well laid at by David Aaronovitch in his column in the Guardian (I’ve been trying to find an online version of his article, if anyone knows where one is please post it)