WASHINGTON, Aug 7 (Reuters) - The U.S. Senate voted to advance a $1 trillion infrastructure package on Saturday but remained on a slow path toward passage with two Republicans openly opposing behind-the-scenes efforts to wrap up work on one of President Joe Biden"s top priorities. In a 67-27 vote demonstrating broad support, senators agreed to limit debate on the legislation, which represents the biggest investment in decades in America"s roads, bridges, airports and waterways. Eighteen of the Senate"s 50 Republicans voted to move forward on the legislation, with Senators John Cornyn and Deb Fischer backing the package for the first time. Lawmakers were working behind closed doors to reach an agreement on amendments that could allow the Senate to complete its work on the legislation as early as Saturday night. Republican Senator Bill Hagerty took to the Senate floor to underscore his opposition to expediting the process, saying the legislation would add to the national debt and set the stage for Democrats to move forward with a separate $3.5 trillion spending package to which Republicans are vehemently opposed. "There"s absolutely no reason for rushing this process," Hagerty, a freshman senator who was former President Donald Trump"s ambassador to Japan, said in a floor speech. "While I believe in hard infrastructure, I cannot participate in doing it this way," he added. Senator Marsha Blackburn, like Hagerty from Tennessee, said she believed the bill "needs a thorough amendment process." The Republican rhetoric appeared to set the stage for a marathon session, after Senate Majority Leader Chuck Schumer vowed to keep the Senate open until lawmakers vote on whether to adopt the bill. "We can get this done the easy way or the hard way. In either case, the Senate will stay in session until we finish our work," Schumer said in a floor speech before the vote. "It"s up to my Republican colleagues how long it takes." With the consent of all 100 senators, the chamber could move through amendments to passage later on Saturday. But without such an agreement, passage could take until Monday or Tuesday. Hagerty, who voted against Saturday"s measure, first registered his opposition to an expedited path after the nonpartisan Congressional Budget Office said on Thursday the legislation would increase federal budget deficits by $256 billion over 10 years. The CBO analysis did not include $57 billion in added revenue that senators estimate Washington would collect over the long term from the economic growth benefits of infrastructure projects. It also did not count $53 billion in unused federal supplemental unemployment funds to be returned from states. OVERDUE INVESTMENTS Passage would be a major victory for Schumer, Biden and a bipartisan group of senators who spent months crafting the package, and would send the bill on to the U.S. House of Representatives. Biden tweeted his support ahead of the vote, saying the "once-in-a-generation investment in our nation"s infrastructure" would create good-paying jobs refurbishing America"s roads, bridges, water systems and electrical grid. "We can"t afford not to do it," the president said. "We can"t just build back to the way things were before COVID-19, we have to build back better." The chamber"s top Republican, Mitch McConnell, also signaled his support before voting for the bill. "Republicans and Democrats have radically different visions these days, but both those visions include physical infrastructure that works for all of our citizens," McConnell said in a speech. "The investments this bill will make are not just necessary, in many cases, they are overdue. Our country has real needs in this area." Lawmakers have been unable to reach agreement on a final batch of amendments that could speed up consideration, leaving the Senate to consider them on a piecemeal basis under rules that require legislation to move forward in stages through a series of procedural votes. Asked how long the process could take, Senator John Thune, the chamber"s No. 2 Republican, told reporters: "Depends how long we spend staring at each other." Reporting by David Morgan and Makini Brice; additional reporting by Idrees Ali; Editing by Scott Malone and Sonya Hepinstall Our Standards: The Thomson Reuters Trust Principles.