WASHINGTON — President Trump promised tax cuts Friday “which will be the biggest in the history of our country” following Senate passage of a $4 trillion budget that lays the groundwork for Republicans’ promised tax legislation.

Republicans hope to push the first tax overhaul in three decades through Congress by year’s end, an ambitious goal that would fulfill multiple campaign promises but could run aground over any number of disputes. Failure could cost the party dearly in next year’s midterm elections.

The budget plan, which passed on a near party-line vote late Thursday, includes rules that will allow Republicans to get tax legislation through the Senate without Democratic votes and without fear of a Democratic filibuster. Nonetheless, the Republicans’ narrow 52-48 majority in the Senate will be difficult for leadership to navigate, as illustrated by the Republicans’ multiple failures to pass legislation repealing and replacing Obama’s health care law.

The final vote on the budget was 51-49 with deficit hawk Sen. Rand Paul of Kentucky the lone opposing Republican vote.

Trump insisted over Twitter on Friday that Paul would be with him in the end on taxes, even though the senator has been critical of the tax package as it’s emerged thus far.

Trump wrote, “The Budget passed late last night, 51 to 49. We got ZERO Democrat votes with only Rand Paul (he will vote for Tax Cuts) voting against……..This now allows for the passage of large scale Tax Cuts (and Reform), which will be the biggest in the history of our country!”

It remains to be seen whether the overhaul will add up to the biggest tax cuts ever. Trump and Republicans have only produced a nine-page framework, leaving plenty of blanks that Congress needs to fill in over the coming months on income-tax brackets and elimination of some favored deductions.

House Speaker Paul Ryan, R-Wis., said Friday the Republicans will add a fourth tax bracket for high-income people to the three originally proposed, but Ryan didn’t say what the tax rate would be for that bracket. Speaking on “CBS This Morning,” Ryan said Republicans are working on the tax rate for “the fourth bracket that the president and others are talking about that we’re going to do.”

The House has passed a different budget, but House Republicans signaled they would accept the Senate plan to avoid delaying the tax measure.

Republicans are looking for accomplishments following an embarrassing drought of legislative achievements despite controlling both chambers of Congress and the White House. Republican lawmakers publicly admit that failure on taxes would be politically devastating with control of the House and Senate at stake in next year’s midterm elections.