A win for the Shiite militant group and its allies in the parliamentary vote would set back US Mideast policy and boost the influence of Hezbollah backers Syria and Iran.
It would also give Hezbollah, which the United States considers a terrorist organisation, a major say in the formation of Lebanon's government for the first time.
But Hezbollah's Christian allies argue that a victory by their coalition will not have such a dramatic impact and will ensure peace in a nation where tensions among the Christian, Sunni and Shiite populations have repeatedly threatened to explode into civil war in the past four years.
They say that involving Hezbollah more deeply in the political process - rather than shunning it - is the only way to bridge the sectarian divides.
Their opponents counter that Hezbollah would be driving Lebanon into the arms of Iran, which could use it as a front in the Islamic republic's confrontation with Israel.
Lebanon has long been a main front in what many see as a zero-sum power struggle between two main camps in the Mideast - the US and its moderate Arab allies Saudi Arabia and Egypt on one side, and Iran and Syria and militant groups like Hezbollah and the Palestinian Hamas on the other. So any change in Lebanon's delicate political balance will resonate beyond its borders.
Washington, which has given around $1 billion to Lebanon's pro-Western government since 2006, has warned it could reconsider aid depending on the election's outcome. Vice President Joe Biden delivered that warning when he visited the country last month.
Neighbouring Israel has also raised the alarm. Vice Prime Minister Silvan Shalom called a win by Hezbollah, which fought the Jewish state in a 2006 war, "very dangerous for the stability of the Middle East, and by that the stability of the entire world."
Hezbollah has tried to strike a moderate tone in the election campaign, in part to ease the concerns of Christians who will likely play a decisive role in the vote. It promises to invite its pro-Western opponents to join a national unity government if it wins.
The proposal shows Hezbollah's concern over a punishing international backlash if it tries to govern Lebanon outright - as happened when the Iranian-backed Hamas won 2006 Palestinian elections, then violently took over the Gaza Strip. Hezbollah itself is only fielding 11 candidates.
The opposing coalition has hammered warnings that a Hezbollah victory will bring Iranian control in Lebanon, reinforce Hezbollah's power as a virtual state-within-a-state and ensure it can keep its considerable arsenal of rockets that it used against Israel in the 2006 war. That resulted in massive retaliation and destruction.
Hezbollah officials maintain the guerrillas' weapons are necessary to defend against Israeli attacks and an election loss would not necessarily weaken the group's military strength. A defeated, embittered Hezbollah might also take a harder line.
The pro-Western coalition opposes Hezbollah keeping its arms and accuses Syria of continuing to interfere in Lebanese affairs and smuggling weapons to Hezbollah even though the two countries established diplomatic relations for the first time last year.
Syria dominated Lebanon for 29 years until it was forced to end its control in the wake of the 2005 assassination of former Lebananes Prime Minister Rafik Hariri. Many in Lebanon blamed Syria, though Damascus denied any role. And US-led international pressure coupled with massive street protests drove the tens of thousands of Syrian troops out of the country a few months after the massive suicide truck bombing that killed Hariri.
The pro-Western coalition, led by Hariri's son, Saad Hariri, supports an international tribunal established in the Netherlands in March to try the killers of his father.
Hezbollah criticizes the tribunal as a tool to damage Syria.
In their campaign, Hezbollah and its allies have promised to end US interference in Lebanon and to bring political and economic reforms. They say a win won't bring isolation or some Iranian-style Islamic state.
For the past year, Lebanon has had a unity government in which pro-Western parties dominate but Hezbollah holds veto power over major decisions.
Hezbollah was brought into the government as a compromise to resolve one of the most explosive moments in Lebanon's long political crisis, when Hezbollah gunmen overran Sunni pro-government neighborhoods of the capital in violent clashes a year ago.
Going into the election, the race for a majority in the 128-seat legislature appears too close to call. In the outgoing parliament, the pro-Western bloc had 70 seats and Hezbollah's alliance had 58.
Hezbollah's coalition includes the Shiite party Amal and a major Christian faction led by former army chief Michel Aoun. Opposing it are the overwhelmingly Sunni Muslim supporters of current majority leader Saad Hariri, allied with several Christian and Druse factions.
Lebanese vote mainly along sectarian or family loyalties. Sunni and Shiite districts around the country are largely locked up, so the battle has been over the Christian districts, where some races are a tossup. There are no reliable, independent polls.
There are some 3.2 million eligible voters out of a population of 4 million. Many are expected to head to polling stations in government buildings and public schools.
Some 50,000 soldiers and police are being deployed to secure balloting. Polls open at 0400 GMT Sunday and close 12 hours later. Early unofficial returns were expected late Sunday and official results as early as Monday afternoon.
Scores of foreign observers including former US President Jimmy Carter will monitor the vote.
The campaign has been bruising, with accusations of vote-buying by both sides, common charges in Lebanese elections. Arrivals at Beirut's international airport have increased sharply, owing to thousands of expatriates flying home to vote - fueling accusations by each side that the other is buying tickets for their supporters abroad.
President Michel Suleiman could ease the political polarisation if neither camp wins a clear majority. Leaders of the pro-Western coalition have said they would accept a unity government if Suleiman plays a major role as arbiter.
The vote is the latest chapter in four tumultuous years for Lebanon that began with Hariri's assassination in 2005. The pro-Western factions swept into power in elections the same year on a sympathy vote. But the government has been virtually paralyzed since by the power struggle with Hezbollah.
