After days of demonstrations against fuel price rises in provincial towns, Muslim Brotherhood supporters joined crowds in Amman on Friday in a rare focus of anger on the king.
"The people want the downfall of the regime", about 3,000 people chanted, in an ominous signal to a U.S.-backed monarchy accustomed to juggling internal rifts between its tribal East Bank and Palestinian citizens, as well as rivalries among its stronger neighbors Syria, Iraq, Saudi Arabia and Israel.
Exposed to the bloody upheaval in Syria, dependent on Iraq for its oil supplies and on Saudi Arabia for funds, Jordan, with its majority Palestinian population, is also sensitive to actions by Israel, which is now bombing Hamas-ruled Gaza.
Instability in Jordan, one of only two Arab countries to sign a peace treaty with Israel, would be alarming for its Western patrons and its conservative Gulf Arab allies.
"Gulf countries must be very worried about any signs of the collapse of the Jordanian monarchy, which would be the first one to fall in the Arab Spring context," said Valerie Yorke, a London-based expert on Jordan.
The kingdom has long relied on Western support and intermittent dollops of Gulf financial aid to survive.
But Saudi Arabia, Amman's main donor, is not known to have provided money since a $1.4 billion infusion in late 2011 to stave off a previous dire economic crisis in the kingdom.
What makes this puzzling is that last year the Saudis had seemed to be trying to draw Jordan into a closer embrace.
In May 2011, the six-nation Gulf Cooperation Council accepted Jordan's application to join, after snubbing it for 15 years, in what was seen as a gesture of monarchical solidarity in face of a wave of popular revolts sweeping the Arab world.
The Saudi-led GCC, which also invited distant Morocco to join, may have wanted Jordanian and Moroccan security help in damping unrest in its own backyard, especially Bahrain, where Saudi and other Gulf troops helped the Sunni Muslim king crush Shi'ite-led pro-democracy protests in March 2011.
Saudi Arabia, eager to counter the influence of Shi'ite heavyweight Iran, was also looking to reorder its alliances after the shock of seeing its longtime Egyptian partner, Hosni Mubarak, overthrown with no U.S. effort to save him.Promises of reform
For now, Jordan is grappling alone with a budget shortfall that prompted Tuesday's fuel subsidy cuts, which the IMF demanded among its conditions for a proposed $2 billion loan.
Anger over the price rises ignited protests similar to those in 1989 when the late King Hussein responded with a political opening that led to a remarkably free election and made Muslim Brotherhood-linked Islamists the biggest bloc in parliament.
Since then, electoral rules have been revised to produce a succession of tame assemblies dominated by the monarchy's tribal, East Bank power base. Political reform projects launched by King Abdullah since he took power in 1999 have faltered.
Gulf Arab rulers, who permit few if any constitutional limits on their own power, might not look kindly on any major political reform or greater freedom of expression in Jordan.
King Abdullah accepted constitutional changes in August that devolved some of his powers to parliament and paved the way for a prime minister chosen by the assembly rather than by him.
But for all his reform talk, there has been no big overhaul of the electoral law to govern an election in January, which the Muslim Brotherhood plans to boycott in protest at perceived discrimination against its mainly urban and Palestinian base.
Liberals and Islamists have long pushed for peaceful change, not revolution, in Jordan, but the latest unrest has spawned slogans that echo those aimed at other Arab rulers seen by some of their people as corrupt, oppressive puppets of the West.
It also reflects the wrath of East Bankers who fear the king might enact reforms at their expense and who resent austerity measures that reduce the patronage, state jobs and other perks that have come their way from the palace in the past.
Rhetorically, the West backs democratic reform, but it also values a monarchy that has long kept Jordan stable in a volatile region, where it provides a buffer on Israel's eastern border.
The United States and its allies are having to come to terms with new realities in Arab republics like Tunisia and Egypt, where Islamists won elections after autocrats were overthrown.
Arab monarchies have survived so far, but their future poses a familiar dilemma for the West, whose desire for reliable allies in the Middle East has frequently outweighed concern for democracy or human rights, as in oil giant Saudi Arabia and in troubled Bahrain, which hosts the US Fifth Fleet.
"The West supported stability rather than change in the region, and in Jordan, over the past 30 or 40 years and paid for it - financially at the time, politically now," Yorke said.
Saudi Arabia and its Gulf allies have helped keep Jordan afloat financially in recent years and the absence of their petrodollars now is keenly felt in a kingdom under stress.
LONDON (Reuters) -- With protesters baying for his overthrow, Jordan's King Abdullah might be wondering why his fellow-dynasts in Gulf Arab states are not providing the cash that could calm the trouble.