JERUSALEM, Nov 15 (Reuters) - Israel's threat to reprise its Gaza invasion of four years ago if its air strikes against Hamas do not end rocket fire from the Palestinian enclave masks important differences between then and now.

Two days into the assault, the absence of the saturated aerial bombing seen at the start of the last Gaza war in 2008 suggested the Israelis were not yet carving safer access points for ground troops.

A cabinet statement on Wednesday spoke only of "improving" national security - acknowledgement that the government has no illusions about crushing the militants once and for all.

There was also a marked difference in casualties, with 13 Palestinian militants and civilians killed a day into the offensive against some 270 in the same period in 2008.

The disparity in scale and pace with its offensive in 2008 reveals much about Prime Minister Benjamin Netanyahu's delicate position.

Israel's previous, centrist government enjoyed broad support in the West and, tacitly, among U.S.-aligned Arab powers. By contrast, the hawkish Netanyahu's listless peacemaking with Hamas's moderate Palestinian rivals has raised hackles in Europe and Washington, and he faces an ever-more hostile Middle East.

Though opinion polls favour Netanyahu for Israel's Jan. 22 election, that lead could bleed should soldiers end up in protracted house-to-house combat in Gaza's warren-like towns.

"In Israel, popular support for a military campaign can be transient," Yoaz Hendel, a military affairs commentator and former Netanyahu spokesman, told Reuters. "I can't see a long-term reoccupation of Gaza taking place before the ballot."

REGIONAL RESONANCE

Then again, Israel is chafing at the sporadic missile salvoes that have disrupted life in its south, compounding the threats proliferating on its Egypt, Syria and Lebanon frontiers.

Nor can Netanyahu afford to look weak against the guerrillas in neighbouring Gaza when he has so long threatened a last-resort war to deny Iran - Israel's distant and militarily formidable arch-foe - the means to develop nuclear weaponry.

Israel appeared to catch Hamas off guard on Wednesday, killing its top military commander Ahmed al-Jaabari in a missile strike and then targeting guerrilla munition dumps in air raids.

But rockets soon started raining down on southern Israel, killing three Israeli civilians in a single blast early Thursday. If such fatalities mount, the government will come under huge pressure to ratchet up the military campaign.

Asked on Wednesday if Israel might send ground forces into Gaza, Brigadier-General Yoav Mordechai said: "There are preparations, and if we are required to, the option of a entry by ground is available."

Retired Mossad spymaster Efraim Halevy warned compatriots against toppling Hamas to prevent more radical Islamists from running Gaza and its impoverished population of 1.7 million.

"If, as everyone hopes, the IDF (Israel Defence Forces) emerges victorious from this round of violence and Hamas emerges beaten and weakened, the time will arrive for the victor to extend its hand to the defeated party in an attempt to engage in practical talks to achieve quiet for both sides," Halevy wrote in Israel's biggest-selling newspaper Yedioth Ahronoth.

"If we fail to do so, the only thing that we will have gained is a temporary increase in the intervals between one round of violence and the next, and nothing more."

Hamas signalled diehard defiance, however. Giving Jaabari a hero's burial, it vowed to redouble its long war on Israel, which was hit by some 200 rockets from Gaza since Wednesday.

Yet, outgunned by the Israelis and shunned by the West as an obstacle to peace, Hamas appealed to Egypt to help, apparently unsatisfied by the recall to Cairo of its envoy from Tel Aviv.

"We await an Arab voice, and especially an Egyptian voice, that is different and does not stop at issuing statements of condemnation but takes practical, serious and substantial steps to force the Occupation (Israel) to rethink before broadening operations in Gaza Strip," Hamas official Mustafa Assawaf said.

EGYPT PRESSURE

Internal pressure on Egypt's Islamist president, Mohamed Mursi, to come to Hamas's aid could build if the Gaza assault proves protracted - as both the combatants anticipate it will.

Israel Hasson, an opposition lawmaker who has represented the Netanyahu government in sensitive talks with Egypt, said the Gaza assault should be pressed regardless of censure from Mursi.

Hasson predicted that Egypt might throw open its border with Gaza, effectively ending the coastal Palestinian enclave's isolation and allowing Mursi's "fighting Hamas brothers" to import and move freely through the Egyptian Sinai desert.

That would deepen Israeli distress over Hamas's military build-up and jihadi threats from the Sinai. But Hasson said Israel, which in the past used Cairo's mediation to secure truces with Hamas, must now itself "set new rules of the game for the new court that has been created" in both Gaza and Egypt.

"Israel is taking this opportunity to make clear that it will defend its vital interests," he said.

"After four years of a policy of accommodation and lulling ourselves, we need to arrive at a policy of deterrence, and even an option of victory, to ensure there will be no more firing (from Gaza) on civilians," Hasson told Reuters.