By David Morgan
WASHINGTON (Reuters) -U.S. House Republicans face a major test this week as the fractured and narrow caucus tries to unify around a plan to advance President Donald Trump's agenda for trillions in tax cuts and new spending on defense and border security.
With only a 218-215 majority in the House of Representatives, Speaker Mike Johnson can afford to lose just one vote on any measure that all Democrats vote against. He faces resistance from as many as a dozen Republicans over a budget resolution that would allow congressional committees to begin crafting full-scale legislation to enact the Trump agenda.

The House Budget Committee was due to take up the measure on Monday, with the possibility of a floor vote as early as Tuesday. But Johnson said timing would also depend on the outcome of Monday night meetings with wavering lawmakers.
"We expect to get it done this week," the Louisiana Republican told reporters in the U.S. Capitol.
"There's a couple of folks who just have lingering questions. But I think all those questions could be answered and we'll be able to move forward," he added. "We're very optimistic. We'll get this thing done."
The House resolution calls for $4.5 trillion in tax cuts -- a concern to lawmakers worried about the nation's growing $36 trillion in debt -- and calls for $2 trillion in cuts to spending -- which have worried some lawmakers that their constituents could lose out on key services.

Republicans in both the House and Senate need to pass the measure to unlock a key part of their strategy: a parliamentary tool allowing them to circumvent the Senate filibuster and opposition from Democrats.
But that is only one feat awaiting lawmakers over the coming weeks. Congress also needs to avert a partial government shutdown after March 14, when funding runs out and then raise the nation's debt ceiling or risk a catastrophic default at mid-year.
Trump commands fealty from congressional Republicans -- most visibly by succeeding in securing Senate approval for all his Cabinet nominees so far, including controversial choices such as vaccine skeptic Health and Human Services Secretary Robert F. Kennedy Jr. and Tulsi Gabbard, his director of national intelligence, who raised concern with past statements seeming to express support for U.S. adversaries.
FREEDOM CAUCUS DEMANDS
The full House Republican caucus has yet to unify around the plan, despite buy-in from the ultraconservative House Freedom Caucus, a group of roughly three dozen hardliners whose demands have plagued a series of House speakers over the past decade.
The Freedom Caucus embraced the resolution after the House Budget Committee increased the target for spending cuts from $1.5 trillion to $2 trillion while preparing the resolution for a floor vote this week. The change has raised concerns among other Republicans over a clause that calls for tax cuts to be pared back to compensate for any shortfall in spending cuts.
The higher target for spending cuts has raised concerns among lawmakers who worry about the impact on food assistance, education grants and the Medicaid health insurance program for the poor, especially for Hispanic communities that helped give Republicans their trifecta of control over the White House and both chambers of Congress.
"Some members are between undecided and 'lean no' on the resolution. Many of us still need clarity from leadership on where these savings will be coming from," said Representative Nicole Malliotakis, one of eight Republicans who expressed their concerns to Johnson in a February 19 letter.
"I need answers to some questions about how we're going to achieve that without hurting Medicaid populations. That's kind of where I'm at," Malliotakis, of New York, said in an interview.
Johnson stressed that the budget resolution is "merely the starting point" for advancing Trump's agenda and that the measure itself does not call specifically for cuts to individual programs including Medicaid.
His conference also remains vulnerable to potential infighting over other hot-button issues including a proposed increase in the federal deduction for state and local taxes and fiscal mavericks who could demand higher cuts in spending.
Representative Victoria Spartz announced on Sunday night that she was a "no" on the measure, saying on social media that she was concerned about spending.
"Almost no one in Congress is serious about cutting spending. They're cowards," Representative Thomas Massie, an independent fiscal hawk and Johnson critic from Kentucky, lamented on the social platform X last week.
Doubts about House Republicans' unity prompted Senate Republicans to enact their own budget resolution as a Plan B ploy last week: a $340 billion measure that covers Trump's border, defense and energy priorities but leaves the thornier issue of tax policy for later in the year.
(Reporting by David Morgan; Editing by Scott Malone and Alistair Bell)