WASHINGTON, July 18 (Reuters) - Republican U.S. lawmakers on Tuesday were left scrambling to salvage their years-long campaign promise to repeal and replace Obamacare after their latest plan sank in the Senate, rattling financial markets and sending the U.S. dollar to a 10-month low.

The setback leaves the party to weigh a repeal-only strategy to buy more time for a possible replacement plan later as President Donald Trump vowed to return with a new proposal.

"As I have always said, let ObamaCare fail and then come together and do a great healthcare plan. Stay tuned!" he tweeted, blaming Democrats and a few members of his own party.

Senate Republicans' second attempt at a healthcare bill collapsed on Monday as two more senators refused to back it, prompting President Donald Trump to urge an outright repeal even as other Republicans sought a shift toward bipartisanship with Democrats.

Republican's healthcare failure spelled uncertainty for Trump's other top agenda items of tax reform and an infrastructure overhaul, leaving the president without any major legislative accomplishments six months into his tenure.

On Monday, Republican Senators Mike Lee and Jerry Moran joined colleagues Susan Collins and Rand Paul in opposing the legislation that would unwind the 2010 Affordable Care Act passed under Democratic former president Barack Obama.

Their refusal, combed with full opposition from Democrats, left Senate Majority Leader Mitch McConnell without enough votes to pass the bill in the 100-member Senate.

Trump said Congress should "start from a clean slate" on a new plan that he said Democrats would join. McConnell said he would try to bring repeal legislation to the Senate floor in coming days, but with a two-year delay in implementation to assure a smooth transition.

Democrats have remained united against Republican's efforts to undo Obama's signature domestic achievement that aimed to reduce the number of people without health insurance and help lower healthcare costs even as they acknowledged changes were needed.

In a statement late Monday, Senate Democratic Leader Chuck Schumer urged Republicans to "start from scratch" with Democrats. Representatives for Schumer could not be immediately reached for comment on Trump's Tuesday tweet putting the blame on "all of the Democrats."

Some Republican and Democratic U.S. governors, who help oversee the joint federal-state Medicaid program for the poor as well as private health insurers, have balked at Republican lawmaker's efforts to undo a law that expanded Medicaid in some states and reduced the number of uninsured people.