Former US House Speaker Paul Ryan has joined the fight against Donald Trump, urging fellow conservatives to reject the former president’s divisive politics and those Republican leaders who emulate him.

Mr Ryan emerged from two years of relative silence to make the remarks on Thursday (local time) during a speech at the Ronald Reagan Presidential Library in California.

He was critical of both Republicans and Democrats, though he saved his sharpest barbs for Mr Trump, who is by most measures the leader of the modern-day Republican Party.

“It was horrifying to see a presidency come to such a dishonourable and disgraceful end,” Mr Ryan said, referring to the deadly attack on the US Capitol that Mr Trump inspired on January 6.

“Once again, we conservatives find ourselves at a crossroads,” Mr Ryan continued.

Former US House Speaker Paul Ryan has spoken out against Donald Trump in a speech. Photo: AAP

“Here’s the reality that we have to face: If the conservative cause depends on the populist appeal of one personality, or on second-rate imitations, then we’re not going anywhere. Voters looking for Republican leaders want to see independence and mettle. They will not be impressed by the sight of yes-men and flatterers flocking to Mar-a-Lago.”

It’s unclear how much impact Mr Ryan’s words will have in the broader fight for the future of the GOP, if any.

Mr Ryan, the 2012 Republican vice-presidential nominee, was among the most respected Republicans in Washington before Mr Trump’s rise. But two years out of office, his open contempt for Trump is not in line with the vast majority of Republican voters and elected officials.

A tiny but growing group of anti-Trump Republicans has struggled to steer the party in a new direction, even as Mr Trump continues to promote the same false claims – that he would have won the 2020 election if not for mass voter fraud – that inspired the Capitol insurrection. At the same time, Mr Trump is openly contemplating another presidential run in 2024.

Those close to Mr Ryan, 51, do not expect him to run for public office again, but they suggest he is paying close attention and remains concerned about the future of the party. The Wisconsin Republican also sits on the board of Fox Corporation, which owns Fox News.

In his speech, Mr Ryan described President Joe Biden’s agenda as “more leftist than any president in my lifetime” and warned of exploding federal spending under the Democrats who control Washington. He lamented the GOP’s interest in culture wars and “identity politics” at the expense of conservative principles.

“Culture matters, absolutely yes, but our party must be defined by more than a tussle over the latest grievance or perceived slight,” he said.

“We must not let them take priority over solutions – grounded in principle – to improve people’s lives.”

The Republican Party had an opportunity to win elections and address critical policy challenges, “as long as they don’t get in their own way”, Mr Ryan said.

“If we fail this test, it will be because the progressive left will have won by default,” he said.

“It will be because the conservative cause … lost its way and followed the left into the trap of identity politics, defining itself by resentments instead of by ideals.

It will be because we mistake reactionary skirmishes in the culture wars with a coherent agenda. It will be because we gave too much allegiance to one passing political figure and weren’t loyal enough to our principles.”

-AAP