(Adds lawmaker comments, background on Democrats)
WASHINGTON, June 27 (Reuters) - Republican leaders were in a fierce push on Tuesday to shore up support for a healthcare bill in the U.S. Senate after the nonpartisan Congressional Budget Office said 22 million Americans would lose insurance over the next decade under the measure.
Vice President Mike Pence is expected to travel to Capitol Hill on Tuesday to join Senate Republicans for a policy lunch before hosting a key conservative senator for dinner.
Senate Majority Leader Mitch McConnell will continue meeting on-the-fence senators who face questions from their governors and state Medicaid officials about potential cuts to the government insurance program for the poor and disabled, lawmakers said.
The CBO analysis on Monday prompted Senator Susan Collins, a key moderate vote, to say she could not support moving forward on the bill as it was written.
At least four conservative Republican senators - Ted Cruz, Rand Paul, Ron Johnson and Mike Lee - said their opposition remained unchanged after the CBO analysis.
Pence will host Lee and other conservative Republican senators at a dinner later on Tuesday, Politico reported, with James Lankford, Tom Cotton and Ben Sasse also invited.
Further, Collins, Paul and Johnson, along with Senator Dean Heller, have all said they will oppose a procedural motion to allow McConnell to move forward and bring the bill up for a vote.
Heller, a moderate Republican up for re-election next year in Nevada, is already facing political fallout after a group started by former campaign aides to President Donald Trump and Pence promised to run ads against him.
The overlapping concerns and competing interests of the lawmakers highlight the balancing act facing McConnell as he tries to unify his party and deliver a legislative win to the president.
Trump and most Republicans in Congress were elected on campaign pledges to repeal and replace Obamacare, Democratic President Barack Obama's signature 2010 law that extended insurance coverage to some 20 million Americans. The pressure is on for them to deliver, now that they control the White House, House of Representatives and Senate.
Senator Angus King, an independent, lamented the lack of presidential leadership to guide the legislation that he said runs counter to Trump's promises to insure everyone, cut costs and protect those with pre-existing conditions.
"He sort of stood on the sidelines and let these bills develop. He celebrated the House bill then said it was mean. I don't think he's getting into the details about what these bills actually do," King told MSNBC.
McConnell's goal was to vote on the measure before the July 4 recess that starts at the end of the week. He can afford to lose just two Republican senators from the party's 52-seat majority in the 100-seat Senate to pass healthcare. Pence would cast a tie-breaking vote.
Moderate senators are concerned about millions of people losing insurance. Conservative senators say the Senate bill does not do enough to repeal Obamacare.
The CBO only assesses the impact of legislation within a 10-year time frame, but it said insurance losses were expected to grow beyond 22 million due to deep cuts to Medicaid that are not scheduled to go into effect until 2025. It would reduce the budget deficit by $321 billion between 2017 and 2026, the CBO estimated.
If the Senate passes a bill, it will either have to be approved by the House, which passed its own version last month, or the two chambers would have to reconcile their differences in a conference committee. Otherwise, the House could pass a new version and bounce it back to the Senate.
"It's the biggest signature issue we have, and it's the biggest promise we've ever made in the modern era," House Speaker Paul Ryan said in a Fox News interview that aired on Tuesday. "We did this in the House. It is now the Senate's turn. I think they're going to do it."
Democrats remained united in opposition, blasting the bill as a tax break for the wealthy that harms the nation's most vulnerable. But they have few options to block it beyond stirring public opposition and mounting pressure on wary Republicans.
(Additional reporting by Yasmeen Abulateb, Amanda Becker, Eric Walsh and Susan Heavey; Writing by Amanda Becker; Editing by Paul Tait and Jeffrey Benkoe)