* Democratic support needed for spending, debt limit bills
* Trump "doesn't keep his word," veteran senator frets
WASHINGTON, Oct 20 (Reuters) - President Donald Trump's disavowal this week of a bipartisan agreement to stabilize Obamacare has further eroded the already minimal trust congressional Democrats had in him, complicating prospects for gaining their cooperation on a range of issues.
Trump's fellow Republicans who control Congress will need at least some Democratic support to pass critical legislation in the coming months to avert a government shutdown and prevent an unprecedented U.S. debt default.
"I've been here with every president since Jerry Ford," said Democratic Senator Patrick Leahy, who was first elected to the U.S. Senate in 1974 and is its longest-serving member.
"I've always been willing to negotiate with anybody because they always keep their word. If you have a president who doesn't keep his word, it makes it more difficult," Leahy said when asked about Trump's interactions with Congress.
Trump initially backed the bipartisan deal announced on Tuesday by Democratic Senator Patty Murray and Republican Senator Lamar Alexander for shoring up insurance marketplaces established under Obamacare for two years by reviving federal subsidies to private insurers that Trump has scrapped. A day later, Trump disowned it.
Going forward, Democratic Senator Jeanne Shaheen said that while more collaboration with Trump is possible, "I think we have to know that we can't count on him."
Trump enraged Republicans in September with his collaboration with top Senate Democrat Chuck Schumer and top House of Representatives Democrat Nancy Pelosi. Trump cut a short-term spending deal with them, blindsiding Republican leaders, and then discussed with them the outlines of a plan to revive protections for young adult immigrants dubbed "Dreamers" brought into the United States illegally as children.
On both healthcare and Dreamers, Trump has shown that while he relishes deal-making, he is quick to row back in the face of a conservative backlash. The president also has sent signals over the past few weeks that he is wavering on working with Democrats to help the Dreamers, making demands that could be a deal-breaker.
Republicans have worked seven years to get rid of Obamacare, Democratic former President Barack Obama's signature legislative achievement, and many of them vehemently opposed the Alexander-Murray deal. It remains unclear whether it will ever come to a vote in the Senate or House.
Because spending legislation requires a super-majority of 60 votes in the 100-seat Senate and there are 52 Republican senators, it cannot pass without at least eight Democrats backing it. Because hard-line House conservatives often oppose spending bills, Democratic support is expected to be needed there as well.
A Dec. 8 deadline is looming for passing the spending measure needed to prevent a shutdown. Democrats could demand a steep price for their support, potentially an Obamacare fix and action to help the Dreamers.
Under a budget resolution passed by the Senate on Thursday night, Republicans need only a simple majority to pass Trump's proposed major tax cuts, though he has urged Democrats to support the overhaul.
'ZIGGING AND ZAGGING'
Schumer, who Trump earlier in the year called the Democrats' "head clown" before their thaw last month, this week vented his frustration about the president, saying he "keeps zigging and zagging so it's impossible to govern."
"This president cannot govern if, whenever the hard right frightens him and says 'jump,' he says 'how high?"' Schumer told reporters on Wednesday.
Schumer and Pelosi dined at the White House with Trump last month, and left feeling as if they had a deal on a proposal to help continue protections for the Dreamers that the president has moved to end effective in March.
Within a day, Trump began distancing himself from that deal, with the White House later floating a series of hard-line immigration "principles," including money for a wall along the U.S.-Mexico border fiercely opposed by Democrats, that have thrown the entire effort into doubt.
Pelosi and Schumer argue their deal with Trump is still salvageable, as the president himself has not called it off.
Pelosi spokesman Drew Hammill acknowledged that the principles "don't help" the effort. But Hammill said that would not stop Pelosi, a former House speaker first elected to the chamber in 1987, from engaging Trump in the future.
"It was always a case-by-case thing. It was never 'the switch was on"' full-time, Hammill said.
There will be plenty of cases to come in the weeks ahead, giving Trump an opportunity to improve his relations with Democrats if that is what he wants, Hammill said.
As Congress and the White House approach the December deadline for settling fiscal affairs, Hammill said Trump can use that opportunity to stabilize Obamacare and shield Dreamers from deportation.
"If he's unable to do that, we're unable to work with him on his priorities," including taxes, Hammill said.
(Reporting by Richard Cowan; Editing by Caren Bohan and Will Dunham)