A leading Republican governor has urged America’s biggest annual gathering of conservatives not to “give up” on the party amid fears that nominating Donald Trump will cause devastating electoral losses for governors and senators.

Scott Walker, who made an ill-fated and short-lived bid for the presidency, told the Conservative Political Action Conference (CPAC) near Washington that Republicans had made significant gains at state levels during Barack Obama’s presidency and this must not be put at risk.

“Some of you may be confused and dare I say even some upset by what’s happening in the presidential election,” the Wisconsin governor said. “I want to offer you some enthusiasm, some optimism today, and tell you no matter what’s happening there, the conservative movement is alive and well in states all across America.”

Describing Republican victories and achievements over the past seven years, he pleaded for grassroots work to continue with an upbeat, optimistic message. “We see positive reforms all across this great country but I just want to remind you that, no matter what you think about what’s happening in the presidential election, you can’t give up.

“We need your help in the states. We’re depending on you at the state and the local level. It’s what our American revival is all about. We’re trying to take America back one state, one community, one person at a time, and that’s why we need your help now more than ever.”

Republicans are bitterly divided over the populist rise of Trump, who has dominated primary elections so far, with some expressing fears that he will alienate voters in crucial swing states, wiping out Republican gains in Congress of recent years. The New York Times reported last week that Senate majority leader Mitch McConnell has assured senators seeking re-election they could run negative ads about Trump to distance themselves from him, and even portray themselves as a necessary check on President Hillary Clinton.

Trump cast a long shadow over the annual CPAC, attended by more than 10,000 conservatives at the Gaylord Convention Center at the National Harbor in Maryland, but his name was not mentioned by speaker after speaker who instead trained their fire on Barack Obama and Hillary Clinton. One speaker called him “the elephant in the room” but again did not utter his name. Trump himself is due to address the conference on Saturday.

Former senator Rick Santorum, another failed presidential candidate who now supports Marco Rubio, said: “There are a lot of conservatives here today, all across this country, who are scared. They’re scared about what’s going on right now. They’re nervous about what’s happening in this presidential race. They’re seeing the conservative movement and the Republican party getting torn up. They’re nervous as all heck.”

As Democrats unite around Clinton, the Republicans seem more divided than ever. New Jersey governor Chris Christie is among the few prominent Republicans to endorse Trump, but others have joined a “stop Trump” effort or remained silent in what commentators say is both a moral and pragmatic choice. There are fears of a deep and lasting split in the party if the New York billionaire becomes its standard bearer.

Nebraska senator Ben Sasse, among the first elected Republicans to declare that he will not vote for Trump, pleaded with the audience to ask how presidential candidates measure up with Ronald Reagan “in a time of constitutional crisis”.

In a dig at the frontrunner’s trademark promise to “make America great again”, he said: “You need to hear from your presidential candidates someone that you would want to sit at the dinner table with your children and extol forth the virtues of an America that is about the greatness of 320 million Americans, not the powers of the federal bureaucracy.

“Do you hear a champion not of tearing more things down, but of building America back up? Because I am anti-establishment, but what we need most of all is not just someone who wants to breathe fire on Washington, but wants to breathe passion into our children for a constitutional recovery, because that’s how we will actually make America great again.”

John Bolton, former ambassador to the UN, declared: “The good news is that when we meet here again next year, Barack Obama will not be president.” But he added darkly that “if we don’t act sensibly”, Clinton will be president.

He added: “Hillary Clinton is a threat to national security, roughly equivalent to Barack Obama. If she doesn’t have the wit to protect her own emails, how can we expect that she’s going to protect us?”