(Reuters) - President Barack Obama will sign a bill to strengthen U.S.-Israeli military cooperation on Friday on the eve of a visit to Israel by his Republican presidential challenger, Mitt Romney.

Obama will seek to stress his commitment to Israel´s security for American Jewish voters at a White House ceremony that appeared timed to upstage Romney, who has accused the president of undermining U.S.- Israeli ties.

Congress passed the legislation, the U.S.-Israel Enhanced Security Cooperation Act, with broad support from Obama´s Democrats and Republicans last week.

"The bill deepens our security cooperation with Israel by expanding our military assistance and providing Israel with access to additional equipment," White House spokesman Tommy Vietor said on Thursday.

Obama, criticized by some of Israel´s U.S. supporters for being too tough on a close ally, wants to shore up his support among Jewish voters, who could prove critical in battleground states like Florida and Pennsylvania in the November 6 election.

Obama received 78 percent of the Jewish vote in the 2008 election, but a nationwide Gallup poll in June showed him down to 64 percent backing versus Romney´s 29 percent.

Romney hopes his trip to Israel will resonate with Jewish voters at home. He will arrive there from London on Saturday and plans to meet Israeli Prime Minister Benjamin Netanyahu, who has had a strained relationship with Obama.

Obama angered many Israelis and their U.S. supporters last year when he insisted any negotiations on the borders of a future Palestinian state begin on the basis of lines that existed before Israel captured the West Bank and Gaza Strip in a 1967 war.

Obama visited Israel as a candidate in the 2008 campaign but has not done so as president. He has insisted security ties with Israel have never been stronger, though he has pressed Netanyahu to hold off on any attack on Iran´s nuclear sites to give diplomacy and sanctions more time to work.

Romney has accused Obama of being too hard on Israel and not strong enough with Iran.

The new bill calls for enhanced cooperation with Israel - the staunchest U.S. ally in the Middle East and a major beneficiary of military aid - on missile defense and intelligence, and increased access to advanced weapons. (© Thomson Reuters 2012. 07/26/12)