WASHINGTON — Vulnerable California Republicans in Congress felt the Earth shift under them Monday as House Speaker Paul Ryan publicly cut his members loose from any obligation of loyalty to the top of the GOP ticket.

Ryan told members of his caucus in a conference call that he would neither campaign with nor defend GOP presidential nominee Donald Trump, providing political cover to Republican House members to denounce the candidate after taped comments by Trump bragging about groping women and Trump’s dismissal of those comments during Sunday night’s debate as “locker-room banter.”

With the Republican Party reeling from the crisis less than a month before election day, three California Republicans considered most at risk of losing their seats denounced Trump after the emergence of the tapes on Friday. Others in safer seats stayed silent.

Freshman Rep. Steve Knight of Lancaster (Los Angeles County), in a tight race against Democrat Bryan Caforio, said Saturday he was “deeply disturbed” by Trump’s remarks and would not vote for him or Democratic nominee Hillary Clinton.

For Rep. Darrell Issa of Vista (San Diego County), the timing of the tape’s release could not have been worse — arriving just hours after Trump had named him to his national security team.

Issa was one of the first House Republicans to endorse Trump, but he issued a statement Saturday on Facebook saying, “This type of behavior has no place in American politics, especially not from those seeking to lead our great nation.” Issa did not, however, withdraw his endorsement, which left an opening for Issa’s Democratic challenger.

“It’s incomprehensible that Darrell Issa still supports this vile man’s candidacy,” Doug Applegate said in a Twitter post Monday.

Rep. Jeff Denham of Turlock (Stanislaus County), challenged by Democrat Michael Eggman, did not return requests for comment to The Chronicle. But in a statement sent to the Modesto Bee, his spokesman Dave Gilliard said Denham was “deeply disappointed” by “Donald Trump’s language toward women and I find it to be beyond inappropriate.” Denham did not withdraw his endorsement of Trump.

Another Republican in a close race to keep his House seat said Saturday the latest revelation reaffirmed why he denounced Trump months ago. In June, Rep. David Valadao, a second-term Republican running against Democrat Emilio Huerta in a heavily Latino district, said he would refuse to support Trump because of his denigrating comments about people “based on their ethnicity, religion, or disabilities.”

On Saturday, the Hanford (Kings County) incumbent tweeted, “I have repeatedly stated I cannot support Trump & this type of disgusting behavior is exactly why.”

Some Republicans, however, remained loyal to Trump, blasting Ryan and other party members for deserting their candidate. Orange County Rep. Dana Rohrabacher called Ryan a coward, both to fellow Republicans on the call and to the Orange County Register.

Ryan is “in a panic,” Rohrabacher told the Register. “It’s not good leadership. ... I think the Republicans who are backing away are gutless. We don’t have to just be concerned about saving House seats. We have to be concerned about saving the United States of America.”

GOP analyst Ford O’Connell said Republican incumbents as a whole face a political quandary. Disloyalty to the nominee threatens to anger Trump’s hard-core loyalists at the party’s base who loathe its “establishment” leaders. Yet support for Trump could alienate the large swathes of general election voters, especially women and minorities, who view Trump with revulsion.

“At least until election day, this is still Donald Trump’s party,” O’Connell said. “Turning against the nominee could cause problems with base voters.”

Kurt Bardella, a former top aide to Issa who now runs his own consulting firm, said in an email that the “wishful thinking” that Republicans down ballot from Trump could insulate themselves from their party’s nominee “has evaporated.”

“At this point, members need to ask themselves if appeasing the fringe of the base is worth being tied to Trump for the rest of their careers,” Bardella said. “If they break with Trump and lose, they still have a path back and can potentially run again. If they stay on the Trump train and lose, there really is no coming back from that.”

Meanwhile, a new NBC/Wall Street Journal poll released Monday vaulted Clinton to an 11-point lead over Trump, a margin so large that if it holds until the election — a big if — it could portend the kind of landslide necessary to put Democrats in the House majority and make House Minority Leader Nancy Pelosi of San Francisco speaker again.

Democrats need to net 30 seats on Nov. 8 to win back control of the House, a huge hurdle given the tiny number of seats in play thanks to Republican gerrymandering and a deepening partisan divide that has all but eliminated moderates in both parties.

Before Trump’s widely panned performance in the first debate last month, his attacks on a former Miss Universe, the lewd tape and Sunday’s debate, Democrats had all but given up hope of recapturing the House. Nathan Gonzalez, a nonpartisan political handicapper at the Rothenberg & Gonzales Political Report, had even lowered his projection for Democratic gains from 10 to 15 seats to seven to 12.

In an interview, Gonzales called the NBC/Wall Street Journal poll “stunning,” but counseled caution.

The poll “was taken over the weekend at arguably the worst possible moment for Donald Trump and Republicans,” he said. Although he rates the NBC/Wall Street Journal poll as one of the best national surveys, “We shouldn’t draw conclusions based on any one survey.”

Moreover, GOP incumbents down ballot have shown what Gonzales described as a remarkable resiliency given the chaos Trump has sown in the Republican Party. Gonzales said that makes him hesitant to conclude that incumbents down ballot from Trump are suddenly vulnerable.

O’Connell predicted Republicans would narrowly maintain House control, if only because Democrats would have to win nearly every vulnerable seat to reach 30.

“It’s still a long haul for Democrats, but then again, we don't know what else could drop,” O’Connell said.

Either way, Republican challengers are making up their minds about Trump.

Justin Fareed, battling Democrat Salud Carbajal for an open seat in Santa Barbara, condemned Trump’s 11-year-old taped remarks. In response to a comment on his Facebook page, he said he had not endorsed anyone in the presidential race.

Scott Jones, running against California’s most vulnerable Democratic incumbent, Ami Bera of Elk Grove (Sacramento County), disavowed Trump after the tape surfaced. Jones called Trump’s actions, if what the presidential candidate said were true, “at best disgusting, and at worst criminal.”

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. Email: clochhead@sfchronicle.com