The United States has warned it was preparing “crippling” and “impactful measures” designed to force Nicolás Maduro from power as Donald Trump rolled out the red carpet for the Venezuelan leader’s challenger, Juan Guaidó.

Guaidó, who has spent the last year battling – so far unsuccessfully – to topple Maduro, arrived at the White House on Wednesday afternoon and was met by Trump.

Earlier, a senior administration official told reporters to expect the moves against Maduro in the next 30 days, without specifying what form they might take.

“We are probably halfway to what maximum pressure [on Maduro] could look like,” they said. “We are only moving in one direction and that is forwards.”

The official said Trump instructed all parts of the US government “to use all of the tools at their disposal to further create stress upon Maduro and his cronies”.

But Guaidó’s tête-à-tête with Trump was overshadowed by Mitt Romney’s announcement that he would vote for the US president’s impeachment.

As that news broke, journalists were abruptly excluded from a planned Oval Office encounter between Trump and Guaidó by the White House press office, without immediate explanation.

The previous evening Trump had used his State of the Union address to offer Guaidó a high-profile show of support.

As Guaidó looked on from the gallery, Trump declared him “the true and legitimate president of Venezuela” leading a “righteous struggle for freedom”.

“Maduro is an illegitimate ruler … but Maduro’s grip on tyranny will be smashed and broken,” Trump vowed to loud applause from both Republican and Democratic lawmakers.

In a clear sign of how Trump sees Venezuela as an important element of his 2020 presidential campaign, he added: “Socialism destroys nations. But always remember, freedom unifies the soul.”

Observers described Trump’s public blessing as a triumph for Guaidó, who defied a travel ban to leave Venezuela last month and is concluding a world tour which also included meetings with Boris Johnson, Emmanuel Macron and Justin Trudeau.

“I think it’s huge for Guaidó,” said Guillermo Zubillaga, a Venezuela specialist at the Americas Society/Council of the Americas. “It erases all the doubts about his leadership … it unites people around him and it emboldens him. I don’t know how this trip could have been more successful.”

Vanessa Neumann, Guaidó’s envoy to the United Kingdom, said Trump’s statement of “very solid” support would also shield Guaidó from arrest when he tried to return to Venezuela in the coming days. The senior administration official said Maduro would face “very significant consequences” if Guaidó was harmed.

But many analysts question what concrete impact Trump’s words will have on Guaidó’s faltering campaign to force out Maduro.

Luis Vicente León, a top Venezuelan pollster, said Guaidó could certainly consider Trump’s backing a victory.

Juan Guaidó acknowledges applause during Donald Trump’s State of the Union address on Tuesday. Photograph: Leah Millis/Pool/AFP via Getty Images

But it was now crucial for Guaidó – whose domestic popularity has slumped since he launched his campaign for change in January 2019 – to return home with concrete results such as increased US pressure on Maduro’s key allies, China and Russia.

“If you return with nothing new, if you cannot convert potential energy into kinetic energy, you fall back into decline. And this is a terrible danger for the opposition’s ability to sustain itself in the future,” said León.

León, the head of the Caracas-based polling group Datanálisis, said Venezuelan politics was like an old-fashioned sugar mill: “It removes your juice and then discards you. If you don’t do something different, you’ll get put through the crusher. And whether you end up as bagazo [pulp] in Caracas or in Miami, [it’s the] same shit.”

Guaidó has offered no hint of what – if any – additional support his allies may have offered during his tour.

But during a rally in Miami on Saturday he told supporters “all options are on the table – but also underneath it too”.

Maduro’s foreign minister, Jorge Arreaza, slammed Trump’s “delusional and arrogant” remarks, dismissing them as part of an attempt to revive his moribund regime change strategy for Venezuela.